Annual fecundity, batch fecundity, and oocyte atresia of Atka mackerel (Pleurogrammus monopterygius) in Alaskan waters

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1 19 Abstract Annual potental fecundty, batch fecundty, and oocyte atresa were estmated for Atka mackerel (Pleurogrammus monopterygus) collected n Alaskan waters durng Atka mackerel were assumed to be determnate spawners on the bass of decreasng fecundty after batch spawnng events. Hstologcal examnaton of the ovares ndcated that oocytes n the vtellogenc stage and hgher had been spawned n the current spawnng season. For an average female of 4 cm, potental annual fecundty was estmated to be 41,994 eggs, average batch sze (.e., batch fecundty) was estmated to be 6689 eggs, and there were 6.13 batches per spawnng season. Atresa was estmated by examnng postspawnng specmens and was found to be substantal. The average amount of atresa for a 4- cm fsh was estmated to be 11,329 eggs, resultng n an estmated realzed fecundty of only 3,664 eggs and 4.64 batches of eggs per spawnng season. Manuscrpt submtted 22 Aprl 24 to the Scentfc Edtor s Offce. Manuscrpt approved for publcaton 15 March 26 by the Scentfc Edtor. Fsh. Bull. 15:19 29 (27). Annual fecundty, batch fecundty, and oocyte atresa of Atka mackerel (Pleurogrammus monopterygus) n Alaskan waters Susanne F. McDermott (contact author) 1 Katherne P. Maslenkov 2 Donald R. Gunderson 2 Emal address for S F. McDermott: Susanne.McDermott@noaa.gov 1 Natonal Marne Fsheres Servce Alaska Fsheres Scence Center 76 Sand Pont Way NE Seattle, Washngton Unversty of Washngton School of Aquatc and Fsheres Scences Box 3552 Seattle, Washngton The reproductve bology of Atka mackerel (Pleurogrammus monopterygus) s characterzed by batch spawnng by females and nest-guardng by males. Ths member of the greenlng famly (Hexagrammdae) s dstrbuted n Russan and Alaskan waters where t s usually found n dense aggregatons and assocated wth areas of fast currents, such as the passes between the Aleutan Islands (Lowe et al., 22). Peak spawnng occurs from July through October n Alaskan waters (McDermott and Lowe, 1997) and from June through September off Kamchatka (Zolotov, 1993). Batches of adhesve eggs are spawned n rock crevces and are guarded by males untl they hatch (Gorbunova, 1962; Zolotov, 1993). Studes wth archval tags have ndcated that males guard nests for 3 4 months (Nchol and Somerton, 22). Batches of eggs n dfferent phases of development were found wthn one nest, ndcatng a promscuous matng system (Zolotov and Tokranov, 1989; Zolotov, 1993; Lowe et al., 23). Atka mackerel females spawn ther annual complement of eggs n several batches over the course of the spawnng season. Zolotov (1993) reported that females spawn an average of three batches of eggs per season and that there s at least a two-week hatus between spawnngs n Kamchat- kan waters. To estmate the number of batches spawned by a female, t s necessary to estmate annual fecundty and batch fecundty. Annual fecundty s defned as the total number of eggs spawned by a female n a sngle year and batch fecundty s defned as the numbers of eggs released at one tme (Nchol and Acuna, 21). Drect estmaton of annual fecundty s possble f the number of eggs to be spawned that season s fxed or determnate (Hunter et al., 1985). Fsh wth determnate fecundty are descrbed as showng a well-defned hatus (or gap) n oocyte-sze dstrbuton between the advanced oocytes that wll be spawned that year and the mmature oocytes that wll not develop untl the followng spawnng seasons (Hunter et al., 1985). Speces that are characterzed by contnuous oocyte development and oocyte-sze dstrbutons have often been descrbed as ndetermnate spawners (Hunter et al., 1985) that have the ablty to develop unyolked oocytes contnually and add them to the standng stock of advanced-yolked oocytes even after spawnng begns. Hunter and Macewcz (21) stated that potental fecundty can be estmated f three key requrements are met: 1) one can dentfy a certan standng stock or sze range of

2 2 Fshery Bulletn 15(1) 17 E W 16 W 55 N 6 N 5 N 55 N 45 N Legend 5 N Samples 1994 Samples W 16 W Fgure 1 Samplng locatons for Atka mackerel (Pleurogrammus monopterygus) collected for estmates of annual fecundty and batch fecundty n n the Aleutan Islands, Alaska. oocytes nto whch no new oocytes are recruted once spawnng begns; 2) females used to estmate potental fecundty have not spawned; and 3) atretc losses are neglgble or can be estmated. Recent studes have shown that many fsh speces reabsorb a substantal number of ther oocytes before spawnng or at the end of the spawnng season a process known as atresa (Hunter and Macewcz, 1985; Kjesbu et al., 1991; Ma et al., 1998) Atretc losses n Icelandc cod (Gadus morhua) and Atlantc herrng (Clupea harengus) occur manly durng the earler phase of vtellogenc development, decrease as oocytes mature, and are neglgble near the tme of spawnng (Harðardóttr et al., 21, Kurta et al., 23). Scombrod fshes (mackerels and tunas) have been descrbed as havng extensve atresa at the end of ther spawnng season (Hunter and Macewcz, 21). Prevous studes (Zolotov, 1993) have descrbed Atka mackerel as determnate spawners. However, no study to date has examned the oocyte-sze dstrbuton or shown that fecundty decreases after spawnng. We examned the oocyte-sze dstrbuton for mature oocytes over tme. Addtonally, all ovares were examned hstologcally to dentfy advanced oocyte stages, presence of postovulatory follcles, and oocyte atresa. We estmated potental annual fecundty and batch fecundty. The number of batches to be spawned was calculated by dvdng total fecundty by batch fecundty. Potental fecundty was studed by comparng prespawnng ndvduals wth those that had spawned at least one batch of eggs n order to determne f advanced oocytes are added throughout the spawnng season. Atresa of mature oocytes was estmated from ovares of postspawnng females to calculate realzed fecundty as a measure of oocytes actually spawned per year. Materals and methods Data collecton All Atka mackerel examned n ths study were ether collected by the Natonal Marne Fsheres Servce (NMFS) fsheres observers aboard commercal fshng vessels durng the commercal Atka mackerel fshery n 1993 or by Alaska Fsheres Scence Center (AFSC) scentsts durng the 1994 NMFS bottom trawl survey of the Aleutan Islands (Table 1, Fg. 1). Durng both years, Atka mackerel were subsampled from ndvdual trawl tows. Collectons were stratfed by fsh sze; no more than fve fsh per sze group were collected n each NMFS statstcal area durng a samplng year. The fork length of each fsh was measured to the nearest centmeter, and each fsh was weghed to the nearest.1 kg. In most cases, the stomach was empted before weghng the fsh. The ovares were excsed and placed n labeled cloth bags n 1% formaln soluton buffered wth sodum acetate (2 g per lter soluton). In order to determne maturty stage, and presence of atresa, all ovares were processed hstologcally. Four ovaran sectons (4-μm thck) were staned

3 McDermott et al: Annual fecundty, batch fecundty, and oocyte atresa of Pleurogrammus monopterygus 21 Table 1 Number of fsh collected by year, month, and Natonal Marne Fsheres Servce (NMFS) statstcal area. The number of samples wth postovulatory follcles,.e., n spawnng or postspawnng condton are shown n parentheses. NMFS statstcal areas Year Month Total 1993 October 4 (4) 5 (5) June July (11) 83 August 1 (9) 1 Total wth hematoxln (Sgma Aldrch, St. Lous, MI) and eosn counterstan (Sgma Aldrch, St. Lous, MI) (H&E). Maturty stages were determned by usng categores developed by McDermott and Lowe (1997). Ovares of prespawnng females were defned by the lack of hydrated oocytes n June and the absence of postovulatory follcles once hydrated oocytes were present n July. Presence of atresa was determned accordng to classfcatons developed by Hunter and Macewtz (1985). For the estmaton of annual fecundty, only females whose ovares had not shown any sgns of releasng oocytes (.e., all hydrated oocytes were wthn ther follcles and ovares contaned no postovulatory follcles) were used. Dfferent stages of postovulatory follcles can be dstngushed n fsh that have spawned multple batches (McDermott and Lowe, 1997). These dfferent stages ndcate that postovulatory follcles persst n ovares longer than the tme perod between batch spawnng events, mnmzng the possblty of ncorrectly classfyng a partly spent fsh as a prespawnng fsh. Prespawnng and partly spent specmens were compared to examne determnate spawnng. Gravmetrc method The gravmetrc method was used to count and measure oocytes n weghed samples of Atka mackerel ovaran tssue (Hunter et al., 1985). In our fecundty analyss, small cross-sectons were excsed from preserved Atka mackerel ovares. The amount of tssue removed was dependent on the sze of eggs present, typcally rangng from.3 g to 2.5 g. The target sample sze was approxmately 1 eggs. The cross-secton was dvded nto wedges n order to ensure a representatve sample of ovaran wall tssue n relaton to the tssue contanng eggs. Ovaran wall tssue weght represents a substantal part of total ovary weght and needed to be represented proportonally n the subsamples. The samples were weghed to the nearest.1 g and allowed to st n water for several hours before handlng to reduce exposure to formaln. Ovary tssue was teased apart manually under a dssectng mcroscope and spread evenly n a 5% soluton of glycerol n a dsh over a grd parttoned by 1-cm squares. The dssectng mcroscope was equpped wth a Javeln Ultrachp CCTV vdeo camera (model JE-7442, Javeln Electroncs, Los Angeles, CA), connected to a computer equpped wth BoScan Optmas 4. scannng software (Boscan Inc., Edmonds, WA). All oocyte stages used n our study were prevously descrbed by McDermott and Lowe (1997). It was possble to dstngush the cortcal alveol oocyte stage (stage 3) from the more advanced oocyte stages (stage 4 [ol droplet stage] and stage 5 [vtellogenc stage]) when usng the dssectng mcroscope and camera mage to examne whole eggs. Oocytes n stage 4 and above showed a dstnct rng around the nucleus, whch was attrbuted to the ol and yolk droplets accumulatng n the oocyte. Stage-3 oocytes dd not show a dstnct rng around the nucleus. However, t was not possble to dstngush ol-droplet oocytes (stage 4) from vtellogenc oocytes or more advanced stages (stage 5+) when examnng whole eggs. Because oocyte development s usually attrbuted to ol-droplet and yolk accumulaton, t was ntally assumed that oocytes below stage 4 would not be spawned n the current spawnng event and therefore would serve as reserve oocytes to be spawned n later spawnng seasons. To determne oocyte-sze composton, all oocytes stage 4 and above were measured along a vertcal axs n each 1-cm square of a grd untl 25 eggs had been measured. All eggs n the sample determned to be oocyte stage 4 or above were then counted manually under the dssectng mcroscope to arrve at an estmate of the number of eggs present per gram of tssue that were oocyte stage 4 and greater. Locaton of tssue samples wthn the ovary We examned sx specmens to determne f locaton of tssue samples wthn the ovary affected estmates of fecundty. Two specmens had ovares n the vtellogenc stage, two had ovares n the early hydraton stage, and two had ovares n the spawnng stage. We took three tssue samples per ovary lobe (sx per specmen). The samples were taken from the anteror, center, and posteror locaton wthn each lobe of the ovary. All eggs wthn the tssue sample that were oocyte stage 4 and above were counted and the frst 1 oocytes were measured. Analyss of varance (ANOVA) (S-plus, 7. for Wndows, Insghtful Corp., Seattle, WA) (Venables and Rpley, 22) was used for ths examnaton wth ovary lobe (left or rght), poston wthn the ovary, and maturty stage as factors. Ths analyss was carred out for the mean number of eggs per gram of tssue and mean egg sze as response varables. The ANOVA ndcated that the tssue sample could be taken from ether lobe of the ovares at any locaton. All further samples were therefore taken from a central locaton wthn ether one of the ovary lobes.

4 22 Fshery Bulletn 15(1) Determnate versus ndetermnate spawnng Drect estmaton of annual fecundty s possble f the number of eggs to be spawned that season s fxed (determnate) and there s a well-defned gap n the oocyte-sze dstrbuton between advanced oocytes that wll be spawned that year and the mmature oocytes. In contrast, ndetermnate spawners have the ablty to develop unyolked oocytes contnually and add them to the standng stock of advanced-yolk oocytes even after spawnng begns. To examne whether Atka mackerel are determnate spawners as prevously descrbed (Zolotov, 1993), we measured ther oocytes to obtan a sze dstrbuton for oocytes over tme. Addtonally, we compared the potental fecundty of prespawnng fsh wth the potental fecundty of fsh that had spawned at least one batch. When all oocytes stage 4 and greater were summed, the tally exceeded the expected number of oocytes to be spawned per year, gven the number of eggs n the frst batch and the prevous estmate of an average of three batches per female (Zolotov, 1993). Ths total number of oocytes led to the hypothess that stage-4 oocytes may be developed, but not spawned, n a gven year and are part of the standng stock of oocytes to be spawned n the followng year. Ths hypothess was supported when we found both ol droplet oocytes and atretc advancedstage oocytes n the spent ovares of nne specmens collected n 1993 (Fg. 2A). The specmens from 1994 were consequently dvded nto two ovary stages: 1 Prespawnng-stage ovares: ovares wth the most advanced oocyte stage of vtellogenc or hydrated oocytes that showed no evdence of havng been spawned,.e., dd not have postovulatory follcles (POFs). 2 Spawnng-stage ovares: ovares that had spawned at least one batch of eggs and therefore contaned POFs. Potental annual fecundty was estmated wth the gravmetrc method and the total number of oocytes were further dvded nto stage-4 and stage-5+ oocytes by usng the methods descrbed below. The data were log-transformed and lnear regressons were ftted by usng S-plus. Ovares of prespawnng and spawnng fsh were compared by testng for dfferences n ntercepts and slopes wth S-Plus (Venables and Rpley, 22). Oocyte frequency dstrbuton A B Mcrometers Fgure 2 Atka mackerel (Pleurogrammus monopterygus) oocyte-sze frequency dstrbuton of (A) stage-4 (clear bars) and advanced atretc eggs (black bars) typcal of a recently spent ovary, measured from whole oocytes (October 1993); (B) an ovary collected on 2 August 1994, wth a hydrated batch of oocytes ready to be spawned (oocytes larger than 18 μm are hydrated). Separaton of stage-4 oocytes from more advanced oocytes To test whether Atka mackerel retan all of ther stage-4 oocytes as a standng stock for the next year s fecundty, t became necessary to separate the stage-4 oocytes from the rest of the more developed oocyte stages and then subtract the stage- 4 oocytes from the total oocyte count. It was not possble to dstngush the oldroplet stage oocytes from oocytes n the vtellogenc stage or mgratory nucleus stage when examnng whole oocytes wth the dssectng mcroscope mage and the Optmas software. Therefore, we examned nne ovares that were recently spent, whch contaned only healthy stage-4 oocytes and atretc hydrated oocytes that could be easly dstngushed (Fg. 2A). The average sze dstrbuton of stage- 4 oocytes was estmated by measurng approxmately 25 stage-4 oocytes per ovary. Ths sze dstrbuton could then be appled to the prespawnng-stage ovares, assumng that stage-4 oocyte-sze dstrbutons n spent ovares were the same as those n prespawnng-stage ovares. To test ths assumpton, the sze dstrbuton from prespawnng-stage ovares n hstologcal preparatons was compared to the sze dstrbuton n postspawnng fsh by usng ANOVA.

5 McDermott et al: Annual fecundty, batch fecundty, and oocyte atresa of Pleurogrammus monopterygus 23 In order to deduct the populaton of stage-4 oocytes from the whole populaton of stage 5+ oocytes n prespawnng-stage ovares, we determned the sze overlap between stage-4 and stage-5 oocytes by examnng hstologcal sectons from sx prespawnng-stage ovares. All oocytes that had been sectoned through the nucleus were measured wth a compound mcroscope and camera system wth Optmas software. We determned that all oocytes smaller than 525 μm were stage-4 oocytes and dd not overlap n sze wth stage-5 oocytes n the prespawnng-stage ovares (Fg. 3A). We used the postspawnng-stage ovares that had only stage-4 oocytes and atretc hydrated oocytes to estmate the proporton of stage-4 oocytes that was less then 525 μm. Ths proporton was then appled to the prespawnng-stage ovares to estmate the number of stage-4 oocytes present. The total number of stage-4 oocytes n the tssue sample for each prespawnngstage ovary was estmated from and c n4 = p (1) g p =, (2) m where n 4 = estmated total number of stage-4 oocytes n a tssue sample of a prespawnng specmen ; c = total number of stage-4 oocytes <525 μm n a tssue sample for a prespawnng specmen ; Percent frequency 3% 25% 2% 15% 1% 5% % 3% 25% 2% 15% 1% p = the estmated proporton of total stage-4 oocytes <525 μm, estmated from ovares of a postspawnng female; g = total number of stage-4 oocytes <525 μm n postspawnng-stage ovares; and m = total number of all stage-4 oocytes n postspawnng-stage ovares. Estmaton of potental annual fecundty and batch fecundty Potental annual fecundty and batch fecundty were calculated by usng prespawnng specmens. Two scenaros were examned for potental fecundty: 5% % A B Mcrometers Fgure Combned sze dstrbutons of (A) stage 4 (clear bars) and stage 5 (black bars) oocytes from hstologcal sldes of sx prespawnng Atka mackerel (Pleurogrammus monopterygus) specmens (1994); (B) stage-4 oocytes from prespawnng (clear bars) and postspawnng (black bars) fsh, measured from hstologcal sldes of 1993 and 1994 Atka mackerel specmen. ˆ no F, = (3) W where F = estmated potental fecundty or batch fecundty for a prespawnng specmen ; n = number of oocytes n the tssue sample of a prespawnng specmen (for potental fecundty total : oocytes of stage 4 and greater; for potental fecundty stage5+, oocytes of stage 5+, for batch fecundty, number of hydrated oocytes); O = total ovary weght (g) for a prespawnng specmen ; and W = weght of tssue sample (g) for a prespawnng specmen. Scenaro 1: potental fecundty total (countng oocytes stage 4 and greater); and Scenaro 2: potental fecundty stage5+ (countng only oocytes stage 5+). The followng equaton was used: Prespawnng-stage ovares that contaned oocytes n the early hydraton stage (stage 7) or greater were examned to estmate batch fecundty. Atka mackerel appear to hydrate one batch of oocytes at a tme and the hydrated oocytes can be clearly separated from less developed oocytes by a gap n oocyte-sze dstrbuton (Fg. 2B).

6 24 Fshery Bulletn 15(1) Length-fecundty relatonshps Length-fecundty relatonshps and ther 95% confdence ntervals were computed for potental annual fecundty ( ˆF ), batch fecundty ( ˆB l ), and the number of atretc oocytes (Êl) by usng nonlnear regresson n S-plus (Venables and Rpley, 22) wth the followng model: Y = al b (4) where y = ˆF, ˆB, Ê ; a and b = fecundty parameters to be estmated; and L = fork length. Oocyte atresa Occurrence of atresa was dvded nto hgh occurrence (more than 1% of oocytes are atretc) and low occurrence (less than 1% of oocytes are atretrc) by examnng vsually all hstologcal sldes. Because ncdence of atresa was low n prespawnng- and spawnng-stage ovares, t was not quantfed further. The occurrence of oocyte atresa n spent oocytes or oocytes that had been spawned was determned by countng the number of atretc oocytes present n the tssue sample of whole oocytes. Atretc eggs were easly dstngushed from nonatretc eggs by ther shrveled appearance, dark to almost black coloraton, and dense texture. In postspawnng fsh, t appeared that all oocytes n an advanced stage were atretc and that stage-4 or smaller oocytes were healthy (Fg. 2A). When the hstologcal sldes of the same specmen were examned for verfcaton of the oocyte stages, t appeared that all atretc oocytes that were counted corresponded to alpha and beta atresa stages (Hunter and Macewcz, 1985). The total number of atretc oocytes for each specmen (E ) was calculated by multplyng atretc oocytes per gram of tssue by total weght of both ovares. Realzed annual fecundty Realzed annual fecundty s defned as the number of oocytes actually spawned per fsh n a gven year. In order to take the occurrence of atresa nto account, a predcted realzed fecundty-at-length was calculated by subtractng predcted atresa-at-length from the predcted potental fecundty-at-length, by usng Rl = Fl El, (5) where ˆR l = estmated realzed annual fecundty at length l; F l = estmated potental annual fecundty at length l; and E l = estmated number of atretc oocytes at length l. Varance (Var) of the predctons at length was calculated as the sum of the varances (estmated wth S-plus) of the predcted values for potental fecundtyat-length and atresa-at-length: Var( R ˆ ) = Var( F ˆ ) + Var( E ˆ ). (6) l l l Annual number of batches spawned The potental number of batches spawned for each specmen was estmated from ˆ ˆ pot F D, = (7) Bˆ where ( ˆD pot ) = the potental number of batches spawned for each specmen ; F = potental fecundty for specmen, and B = batch fecundty for specmen. Batch number for realzed fecundty for specmen was calculated by usng the predcted proporton of atretc eggs per length category and applyng t to each specmen: D real atr F p l ( 1 = l ), (8) B where atr El pl =, (9) Fl where D real l = the batch number per specmen at length l; p atr l = the predcted proporton atretc at length l; E l = the predcted number of atretc eggs at length l; and F l = the predcted potental fecundty at length l. The relatonshp between batch number and length was examned for both potental and realzed fecundty wth lnear regresson (S-plus). Results Data collecton Nne samples were collected after spawnng events n October 1993 n NMFS statstcal areas 542 (n=4) and 62 (n=5) (Table 1, Fg. 1). One hundred-ffty fsh were collected from June through August n 1994 n NMFS statstcal areas 541 (n=57), 542 (n=49), and 543 (n=44). Most (13) of these fsh were n prespawnng condton,.e., contaned no postovulatory follcles, and 2 had spawned at least one batch of eggs. Because the fsh were collected durng the NMFS Aleutan survey, whch systematcally moves from east to west along the Aleutan archpelago, all specmens n area 541 (Eastern Aleutan Islands) were consdered prespawnng fsh Spawnng commenced n late July n area 542 and by the mddle of August all fsh collected showed sgns of spawnng.

7 McDermott et al: Annual fecundty, batch fecundty, and oocyte atresa of Pleurogrammus monopterygus 25 Locaton of tssue samples n ovares The effect of ovary lobe locaton or poston wthn the ovary was not sgnfcant for ether average egg sze (P=.7,.43, respectvely) or mean number of eggs per gram of ovary tssue (P=.7,.53, respectvely). Maturty stage of ovary had a sgnfcant effect on both eggs per gram (P<.1) and average egg sze (P<.1). Separaton of ol-droplet stage oocytes (stage 4) from more advanced oocytes There was no sgnfcant dfference n mean egg sze for stage-4 oocytes n pre- and postspawnng fsh (P=.59)(Fg. 3B). We determned that all oocytes smaller than 525 μm that were measured were stage-4 oocytes (Fg. 3A). The proporton of stage-4 oocytes less than 525 μm ( p ) was.755 based on samples of postspawnng fsh n 1993, wth g = 1592 (the number of stage-4 oocytes that measured smaller than 525 μm), and m = 211 (the total number of stage-4 oocytes measured). Ths proporton was treated as a constant when estmatng the number of oocytes n stage 5 and larger. Frequency Mcrometers 8/2/1994 7/31/1994 7/3/1994 7/27/1994 7/24/1999 7/19/1994 7/18/1994 7/8/1994 7/6/1994 6/2/1994 6/19/1994 6/18/1994 6/15/1994 Fgure 4 Atka mackerel (Pleurogrammus monopterygus) oocyte-sze frequency dstrbuton summarzed by date for Oocyte-sze frequences were averaged per day. Date sampled Determnate versus ndetermnate spawnng Wth the excepton of hydrated eggs, Atka mackerel are characterzed by a contnuous oocyte-sze dstrbuton much lke that of an ndetermnate spawner (Fg. 4). Atka mackerel ovares contan several stages of developng oocytes throughout the spawnng season. However, the length-fecundty relatonshp for the total number of oocytes decreased after the fsh had spawned ther frst batch (Fg. 5A) and there was a sgnfcant dfference n ntercept and slope (P=.1 and.1, respectvely). When the length-fecundty relatonshp for oocytes n stage 5+ was examned, the potental fecundty also decreased after spawnng (Fg. 5B), wth a sgnfcant dfference n ntercept and slope (P=.3 and P=.9, respectvely). The numbers of stage-4 oocytes dd not appear to dffer substantally before and after spawnng for fsh of comparable sze (Fg. 5C) although the slope of the length-fecundty relatonshp dffered sgnfcantly (P=.12). Furthermore, there was no reducton n the number of stage-4 oocytes at comparable fsh lengths for the nne spent fsh collected late n the spawnng season (October) versus prespawnng and spawnng fsh collected n June August. Oocyte atresa Incdence of oocyte atresa n prespawnng and spawnng fsh was found to be low. Only 15 of the 15 ovares examned showed any sgns of atresa. Of these15 ovares, hgh occurrence of atresa (more than 1% of oocytes affected) was found n 3, and low occurrence of atresa (less than 1% of oocytes affected) was found n 12. Therefore, atresa n ovares of pre- and postspawnng fsh was not quantfed further. Atresa was found to be present n all of the nne ovares of postspawnng fsh examned n the study and the number of atretc oocytes corresponded roughly wth the number of ooccytes spawned n a batch (batch sze). Ths fndng ndcates that Atka mackerel may use atresa to regulate reproductve output at the end of the spawnng season. Estmates of annual fecundty and batch fecundty Length-fecundty parameters (a and b) for predcted fecundtes for scenaro 1 (oocytes stage 4) and scenaro 2 (oocytes stage 5), as well as estmates of batch fecundty and atretc eggs are shown n Table 2 wth ther respectve standard errors. Predcted potental fecundty for both scenaros s shown n Fgure 6A. The length-fecundty relatonshp for oocytes at stage 5+ s the most realstc because t appears that most stage-4 oocytes wll not be spawned n the current season. Predcted potental and realzed fecundty for stage 5+ oocytes are llustrated n Fgure 6A wth ther respectve confdence ntervals. Because the estmates of potental fecundty were treated as data n the lengthfecundty regressons, the varance estmates and confdence ntervals are conservatve. Data and model ft for the relatonshp of batch fecundty to length s shown n Fgure 6B, and the data and model ft for the number of

8 26 Fshery Bulletn 15(1) Table 2 Atka mackerel length-fecundty coeffcents derved wth the equaton F = a L b, where F = fecundty and L= fsh fork length (cm). SE= standard error of the mean. CI = 95% confdence ntervals for predcted values of the mean. n = number of fsh n sample. Constants a b Estmate for Fecundty type Estmate SE Estmate SE n 4-cm female (±95% CI) Potental fecundty total ,321±2535 Potental fecundty stage ,994 ±261 Number of atretc eggs ,329 ±882 Batch fecundty ±577 Realzed fecundty total 37,992±7873 Realzed fecundty stage5+ 3,664 ±773 Mean value SD Batch number potental stage ±2.64 Batch number potental total ±2.98 Batch number realzed stage ±2.1 Batch number realzed total ±2.25 atretrc eggs n the ovary by length category s shown n Fgure 6C. Average number of batches correspondng to potental and realzed fecundty are shown n Fgure 6D. The average fork length for females durng the 1994 NMFS survey was 4 cm (Lowe and Frtz, 1994). Potental fecundty (stage 5+) was estmated to be 41,994 eggs, and realzed annual fecundty was estmated to be 3,664 eggs for a 4-cm female (Table 2). Batch fecundty for a 4-cm female was estmated to be 6689 eggs and the number of batches spawned showed no statstcally sgnfcant relatonshp wth length for both potental (P=.35) and realzed (P=.75) fecundty (Fg. 6D). The estmated number of batches produced annually per female was calculated by usng the grand mean and standard devaton for all specmens combned. The average number of batches of eggs for potental and realzed fecundty was estmated at 6.1 and 4.5 batches per year, respectvely, wth correspondng 95% confdence ntervals of ±2.64 and ±2.21 batches. Ths result would ndcate that the average female has the potental to spawn about 6 batches, but may reabsorb 1 2 batches of eggs at the end of the season. Dscusson The reproductve strategy of Atka mackerel and other hexagrammds has been characterzed by low fecundty and large eggs, hgh parental care, and by larvae hatched at an advanced stage of development (Gorbunova, 1962). Potental fecundty for an average female Atka mackerel of 4 cm was 41,994 eggs for oocytes at stage 5+ and s hgher than the estmates for Kamchatkan waters by Gorbunova (1962), who gave a range between 5 and 31, eggs for a 52-cm female. Zolotov (1993) estmated potental fecundty to be 38,7 eggs (vtellne oocytes plus hydrated oocytes n frst batch) off Kamchatka, whch s also slghtly lower than our estmate. Our batch fecundty estmate of 6689 eggs for an average female of 4 cm s smlar to Zolotov s (1993) estmate of 693 for a fsh of the same sze. Atka mackerel are characterzed by a contnuous sze dstrbuton of oocytes much lke that of ndetermnate spawners. However, Atka mackerel have a dstnct spawnng season (July October) durng whch they spawn batches contanng a large number of eggs (Zolotov, 1993; McDermott and Lowe, 1997). Ths spawnng pattern dffers from that of typcal ndetermnate spawners that spawn a small number of eggs (n relaton to the total number present) almost contnuously for many months, as characterzed by the northern anchovy (Engrauls mordax) (Hunter et al., 1985). Atka mackerel seem to le somewhere n the mddle of determnate versus ndetermnate spawnng fsh. It appears that there s a potental to develop and spawn up to seven batches f all stage-4 oocytes are developed and spawned. Most of the stage-4 oocytes stll remanng n the ovary durng the spawnng phase probably consttute the pool of oocytes to be spawned n the next season. However, those oocytes could be a reserve to be developed and spawned n hghly productve years (.e., durng favorable spawnng condtons). Estmates of fecundty based on oocytes at stage 5+ may be conservatve f some of the stage-4 oocytes are

9 McDermott et al: Annual fecundty, batch fecundty, and oocyte atresa of Pleurogrammus monopterygus 27 Numbers of all oocytes A Numbers of stage-5+ oocytes B C Number of stage-4 oocytes Fsh length (cm) Fgure 5 Fecundty of Atka mackerel (Pleurogrammus monopterygus) at the prespawnng (clear trangles) and spawnng (sold squares) phase for (A) total number of oocytes at stage 4 and above, (B) oocytes at stage 5+, and (C) for oocytes at stage 4 only. Regresson lnes for prespawnng and spawnng fsh are ndcated by sold and dashed lnes, respectvely. developed and spawned late n the spawnng season. However, knowledge of batch sze, length of spawnng season, and the spawnng nterval between batches (1 2 weeks) (Gorbunova, 1962; Zolotov, 1993; McDermott and Lowe, 1997), gves us confdence that our estmate of fecundty s realstc. Gven that there are 14 days between batches, a 75-day ncubaton perod (Lowe et al., 23; McDermott, 23), and that 4.6 batches are spawned annually, the spawnng season would be 12 days, whch s smlar to that observed n the present study. Conclusons Prespawnng Atka mackerel dd not show much atresa durng egg development; however hgh levels of atresa became apparent when ovares of postspawnng fsh

10 28 Fshery Bulletn 15(1) were examned. Examnaton of ovares of prespawnng females alone would lead to the assumpton that the potental fecundty s close to the realzed fecundty. Atresa n postspawnng Atka mackerel has been Potental fecundty Batch fecundty A B descrbed n earler studes (Gorbunova, 1962; Zolotov, 1993), where 4 5 atretc eggs per ovary and the prolonged presence of atretc eggs n the ovares before reabsorpton have been reported. However, ths s the frst study where atresa from recently spent ovares has been ncluded n the fecundty analyss n a quanttatve manner. When recently spent ovares were examned, t appeared that atresa had a major effect on potental fecundty and that an average of 1 2 batches were reabsorbed at the end of the spawnng perod n the ovares examned. It should be noted that specmens wth ovares n the prespawnng phase and specmens wth recently spent ovares were collected n dfferent years. Atresa s lkely to be related to feedng ecology, avalablty of approprate substrate and envronmental condtons for spawnng, and could potentally change from year to year. Interannual varablty of atresa should be examned n the future. Number of atretc oocytes Batch number C D Length (cm) Fgure 6 (A) Model ft and 95% confdence ntervals for potental fecundty of Atka mackerel (Pleurogrammus monopterygus) at length for total number of oocytes (sold lnes), stage-5+ oocytes (short dashed lnes), and realzed fecundty (long dashed lnes). (B) Data and model ft (lne) for Atka mackerel batch fecundty by length. (C) Data and model ft for number of atretc oocytes n spent ovares of Atka mackerel by length. (D) Number of batches spawned by length for potental fecundty (sold trangles) and realzed fecundty (open squares) and the estmated populaton means (sold lne for potental fecundty and dashed lne for realzed fecundty) for Atka mackerel. Acknowledgments We thank G. Thompson and J. Ianell for ther statstcal advce and F. Morado for hs advce and assstance wth hstologcal work. We also thank all the observers and scentsts who collected specmens and the commercal fshng ndustry for provdng samplng platforms. G. Stauffer, B. Mller, A. Hollowed, S. Lowe, and S. Hnckley helped greatly by crtcally revewng drafts of ths paper. Ths research was supported by the Jont Insttute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperatve Agreement no. NA67RJ155 (contrbuton no. 986). Lterature cted Gorbunova, N. N Spawnng a nd development of greenlngs (famly Hexagrammdae). In Greenlngs: taxonomy, bology, nteroceanc transplantaton (T. S. Rass, ed.), p Tr. Inst. Okeanol., Akad. Nauk. SSSR 59: In Russan (translated by Isr. Program. Sc. Trans., 197). [Avalable from Natl. Tech. Inf. Serv., Sprngfeld, VA, as TT S.] Harðardóttr, K., O. S. Kjesbu, and G. Martensdottr. 21. Atresa n Icelandc cod (Gadus morhua L.) pror to and durng spawnng. Report of the workng group on modern approaches to assess maturty and fecundty of warmand cold-water fsh and squds (O. S. Kjesbu, J. R. Hunter, and P. R. Wthames, eds.), 14 p. Insttute of Marne Research, Bergen, Norway.

11 McDermott et al: Annual fecundty, batch fecundty, and oocyte atresa of Pleurogrammus monopterygus 29 Hunter, J. R., N. C. H. Lo, and R. J. H. Leong Batch fecundty n multple spawnng fshes. In An egg producton method for estmatng spawnng bomass of pelagc fsh: applcaton to the northern anchovy, Engrauls mordax (R. Lasker, ed.), p U.S. Dep. Commer., NOAA Tech. Rep. NMFS 36. Hunter, J. R., and B. J. Macewcz Rates of atresa n the ovary of captve and wld northern anchovy, Engrauls mordax. Fsh. Bull. 83: Improvng the accuracy and precson of reproductve nformaton used n fsheres. Report of the workng group on modern approaches to assess maturty and fecundty of warm-and cold-water fsh and squds (O. S. Kjesbu, J. R. Hunter, and P. R. Wthames, eds.), 14 p. Insttute of Marne Research, Bergen, Norway. Kjesbu, O. S., J. Klungsøyr, H. Kryv, P. R. Whtthames, and M. Greer Walker Fecundty, atresa, and egg sze of captve Atlantc cod (Gadus morhua) n relaton to proxmate body composton. Can. J. Fsh. Aquat. Sc. 48: Kurta, Y., S. Meer, and O. S. Kjesbu. 23. Oocyte growth and fecundty regulaton by atresa of Atlantc herrng (Clupea harengus) n relaton to body condton throughout the maturaton cycle. J. Sea Res. 49: Lowe, S. A., and L. Frtz Atka mackerel. In Stock assessment and evaluaton report for the groundfsh resources of the Berng Sea/Aleutan Islands regons, North Pacfc Fsheres Management Councl, P.O. Box 13136, Anchorage, Alaska, Lowe, S., J. Ianell, H. Zenger, and R. Lauth. 23. Stock assessment of Aleutan Islands Atka mackerel. In Stock assessment and evaluaton report for the groundfsh resources of the Berng Sea/Aleutan Islands regons, p North Pacfc Fsheres Management Councl, P.O. Box 13136, Anchorage, Alaska Lowe, S., J. Ianell, H. Zenger, and R. Reuter. 22. Atka mackerel. In Stock assessment and evaluaton report for the groundfsh resources of the Berng Sea/Aleutan Islands regons, p North Pacfc Fsheres Management Councl, P.O. Box 13136, Anchorage, Alaska Ma, Y., O. S Kjesbu, and T. Jørgensen Effects of raton on the maturaton and fecundty n captve Atlantc herrng, (Clupea harengus). Can. J. Fsh. Aquat. Sc. 55:9 98. McDermott, S. F. 23. Estmatng abundance of a patchly dstrbuted fsh, Atka mackerel, Pleurogrammus monopterygus. Ph.D. dss., 149 p. Unv. Washngton, Seattle, WA. McDermott, S. F., and S. A. Lowe The reproductve cycle and sexual maturty of Atka mackerel, Pleurogrammus monopterygus n Alaska waters. Fsh. Bull. 95: Nchol D. G., and E. I. Acuna. 21. Annual batch fecundtes of yellowfn sole, Lmanda aspera, n the eastern Berng Sea. Fsh. Bull. 99: Nchol, D. G., and D. A. Somerton. 22. Durnal vertcal mgraton of the Atka mackerel Pleurogrammus monopterygus as shown by archval tags. Mar. Ecol. Prog. Ser. 239: Venables, W. N., and B. D. Rpley. 22. Modern appled statstcs wth S-plus, 495 p. Sprnger-Verlag Inc., New York, NY. Zolotov, O. G Notes on the reproductve bology of Pleurogrammus monopterygus n Kamchatkan waters. J. Ichthyol. 33:(4): Zolotov, O. G., and A. M. Tokranov Reproductve ecology of hexagrammdae and cottdae n the Pacfc waters of Kamchatka. J. Ichthyol. 3: