Offshore distribution of Hector's dolphins at Banks Peninsula, New Zealand: Is the Banks Peninsula Marine Mammal sanctuary large enough?

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1 New Zealand Journal of Marine and Freshwater Research ISSN: (Print) (Online) Journal homepage: Offshore distribution of Hector's dolphins at Banks Peninsula, New Zealand: Is the Banks Peninsula Marine Mammal sanctuary large enough? Elisabeth Slooten, William Rayment & Steve Dawson To cite this article: Elisabeth Slooten, William Rayment & Steve Dawson (2006) Offshore distribution of Hector's dolphins at Banks Peninsula, New Zealand: Is the Banks Peninsula Marine Mammal sanctuary large enough?, New Zealand Journal of Marine and Freshwater Research, 40:2, , DOI: / To link to this article: Published online: 29 Mar Submit your article to this journal Article views: 452 View related articles Citing articles: 12 View citing articles Full Terms & Conditions of access and use can be found at

2 New Zealand Journal of Marine and Freshwater Research, 2006, Vol. 40: /06/ The Royal Society of New Zealand Offshore distribution of Hector's dolphins at Banks Peninsula, New Zealand: is the Banks Peninsula Marine Mammal sanctuary large enough? ELISABETH SLOOTEN 1 WILLIAM RAYMENT 1, 2 STEVE DAWSON 2 Departments of Zoology 1 and Marine Science 2 University of Otago P.O. Box 56 Dunedin, New Zealand Liz.Slooten@stonebow.otago.ac.nz Abstract Aerial surveys of Hector's dolphins were carried out to evaluate the effectiveness of the Banks Peninsula Marine Mammal Sanctuary. In summer, the proportion of sightings inside the 4 nautical mile offshore boundary of the sanctuary was 79%. This dropped to just over 35% in winter. These estimates were used in a population viability analysis to determine whether the sanctuary needs to be extended to reduce dolphin bycatch to sustainable levels. We followed the standard procedure for setting limits on marine mammal bycatch in the United States to calculate a bycatch limit of 1.6 or 2.8 dolphins per year (depending on whether the sanctuary population is included). If the offshore boundary was extended to 15 nautical miles, the sanctuary would need to be extended alongshore north and south by more than 30 nautical miles to reduce bycatch to 2.8, or north and south by 60 nautical miles to reduce bycatch to 1.6 dolphins per year. Keywords aerial survey; Hector's dolphin; Banks Peninsula Marine Mammal Sanctuary; gillnet entanglement; bycatch; sustainability; effective survey design M04148; Online publication date 3 May 2006 Received 1 November 2004; accepted 13 January 2006 INTRODUCTION Hector's dolphins are found only in New Zealand waters and are endangered as a result of entanglement in gill nets (Hilton-Taylor 2000). The species is divided into two subspecies; North Island Hector's dolphin (Cephalorhynchus hectori maui ; also known as Maui's dolphin) and South Island Hector's dolphin (C. hectori hectori) (Baker et al. 2002). Since 1988, two protected areas, in which gill netting is illegal or strongly restricted, have been created to reduce the level of bycatch of Hector's dolphins. The protected areas are located at Banks Peninsula, on the east coast of the South Island, and between Maunganui Bluff and Pariokariwa Point on the west coast of the North Island. Studies of the distribution of dolphins in these protected areas are vital to evaluate their likely effectiveness. One of the most important issues in the design of any protected area is where to place the boundaries. This usually involves studies of the distribution of the species in question, to ensure that a local population is protected, or that a sufficient proportion of the population is protected to reduce impacts to sustainable levels. The distribution of the impact, or more particularly the overlap between the impact and the affected species, is also of interest but is often not as important. This is especially so for fishing interactions, as fishing is typically very dynamic and current activity is not necessarily a good indication of the future distribution of fishing effort (see Murray et al. 2000). In addition, creation of a protected area can act to shift fishing effort to nearby areas, thus shifting rather than solving the bycatch problem (see Slooten et al. 2000). For most species, distribution of threatened populations is readily studied by direct survey methods (e.g., Harlow & Biciloa 2001; Kinnaird et al. 2003). For marine mammal populations this is usually done via boat or aerial surveys. Data on movements of individual animals (e.g., from electronic tags) can be of interest in their own right, but may reveal very little about the distribution of the population as a whole.

3 334 New Zealand Journal of Marine and Freshwater Research, 2006, Vol. 40 Black Birch Creek jf Leithfield Beach/ / Fisheries area ZO Rakaia River í^-^^^. ff J j Rangitata River ^ y^ ^ ^ X s*^ ^"líaiks Peninsula"*^ - ^ Marine Mammal Sanctuary Timaru Á Wz. re io A Lagoon /^ Fisheries area Ï2 OaiTiafj ft 17^E nautical miles (13.5 km) Fig. 1 Location of fisheries areas 20 and 22, existing sanctuary boundaries, and coastal boundaries needed to reduce annual bycatch to 1.6 (Black Birch Creek to Wainono Lagoon) or 2.8 dolphins (Leithfield Beach to Rangitata River). 4ÍS- The Banks Peninsula Marine Mammal Sanctuary (Fig. 1) was created in 1988 (Department of Conservation 1988; Dawson & Slooten 1993). Continued bycatch of dolphins in gill nets immediately outside the sanctuary (Starr & Langley 2000), led us to examine more closely the offshore distribution of the population and how it varies seasonally. Past surveys at Banks Peninsula indicated that Hector's dolphins come closest to shore during the summer months and most are found within 4 nautical miles (1 nautical mile = km) of the coast (Dawson & Slooten 1988). During winter, the dolphins appear to be more dispersed and a smaller proportion is found close to shore. Dawson & Slooten (1988) surveyed 38 offshore transects from the coast out to 5 nautical miles offshore, before the sanctuary was created. Of the dolphins within 5 nautical miles from shore, 45.5% were found within the first 800 m from shore during summer, whereas in winter that proportion dropped to 25% (Dawson & Slooten 1988). The offshore extent of these surveys was limited by the survey platform, a 4 m inflatable boat. The sanctuary boundaries were set in part on these research data and the locations of dolphin captures, and in part on the perceived needs of fishers (Dawson & Slooten 1993). In this paper, we describe results of a set of aerial surveys designed to quantify the offshore distribution of Hector's dolphins at Banks Peninsula. We also incorporate these data into a simple population viability analysis (Burkhart & Slooten 2003) to determine whether the boundaries of the sanctuary need to be extended if dolphin bycatch is to be reduced to what would be the maximum permitted human impact under United States law. The United States National Marine Fisheries Service has developed a model to calculate "quotas" for humancaused mortality on cetaceans and pinnipeds (Barlow et al. 1995; Wade 1998). The model requires a 95% or better probability that: (1) populations starting

4 Slooten et al. Hector's dolphin offshore distribution 335 at the maximum net productivity level (MNPL, assumed to be half of the original population size, carrying capacity or K) stay at or above that level after 20 years; and (2) populations starting at 30% of K recover to at least MNPL after 100 years. This model has been used to set maximum allowable levels of fisheries bycatch for New Zealand sea lions (Manly & Walshe 1999). MATERIALS AND METHODS Aerial line-transect surveys were carried out in February and June These months were chosen on the basis of information from extensive line-transect boat surveys and photo-id surveys in the Banks Peninsula area (e.g., see Slooten et al. 1992, 1993; Bräger 1998; Dawson et al. 2004) to characterise the summer and winter distribution of Hector's dolphins. These surveys showed that dolphin densities close to shore are high in summer (December-February) and lower in winter (June- August). Placement of transect lines followed design principles set out in Dawson et al. (2004) and Slooten et al. (2004). Briefly, the Banks Peninsula Marine Mammal Sanctuary was divided into four sections of relatively straight coastline, and a straight baseline was drawn along each section of the coast. Transects were placed at 45 to the baseline of each section, with the coastal start point of the first transect in each section chosen randomly. The remaining lines were plotted at 4 nautical mile intervals, so that survey effort was approximately uniform out to 15 nautical miles offshore (Fig. 2). We laid out an alternative set of transect lines at 90 to the first set, which were flown if sighting conditions (sun glare in particular) favoured them. The survey platform was a Partenavia P-68; a twin-engine, 6-seater, high-wing aircraft fitted with bubble-windows to allow the rear observers to see the track line directly beneath the aircraft. We flew transects at an altitude of 500 feet (152 m) at a speed of c. 100 knots (185 km/h). Transect lines were navigated using a Garmin GPS11 + Global Positioning System (GPS), with transect waypoints previously loaded into the memory. The direction in which transect lines were flown was decided in the field according to glare at that time. All survey effort was completed in sea states of Beaufort 3 or less. Glare was scored on a scale of 0 to 4 (0 = no glare). Survey effort was discontinued if observers were not confident they could see all dolphin groups close to the trackline. Sampling effort was started no sooner than 30 min after sunrise and completed no later than 30 min before sunset. Aerial surveys were conducted in passing mode (Buckland et al. 1993), meaning that the course of the aircraft was not diverted when a sighting was made. This is possible because Hector's dolphins are easy to identify from the air and their typically small group size makes them easy to count (Slooten et al. 2004). The survey team consisted of four observers (two on each side of the aircraft) and the pilot. Flights were typically 2-4 h in duration. To reduce the risk of bias and fatigue, observers swapped positions diagonally between flights (i.e., the observer in the front right position on the first flight would be in the rear left position on the next flight). Observers wore headphones to receive instructions from the survey leader, but did not communicate when "on effort". Wearing headphones, and the layout within the aircraft itself, ensured that observers gained no visual or acoustic cues from each other. Information about sightings was shared only when "off effort" between transect lines. When a group of Hector's dolphins was sighted, the observer measured the downward angle to the group perpendicular to the aircraft's track using a handheld inclinometer (Suunto PC5/36D PCB). Sighting details were dictated into personal dictaphones, with the time of the sighting noted to the second from digital clocks that were synchronised with local time according to the GPS. A GPS-linked Hewlett- Packard 200LX palmtop computer with customwritten software was used to record starts and ends of transect lines, details of effort (via recording a GPS fix every 20 s) and sighting conditions (noted at the start of each transect, and whenever they changed). The time of each sighting was later used to locate positions of sightings on transects by interpolation between stored GPS fixes. To familiarise the pilot and observers with survey protocols we conducted several training flights before each survey, achieving at least 80 sightings before the start of each survey. These data were used for training only, and are not analysed here. For more information on survey methods and design, see Dawson et al. (2004) and Slooten et al. (2004). After plotting the Hector's dolphin sightings, we counted the number of individuals inside and outside the 4 nautical mile sanctuary boundary in summer and winter. We used these distribution data to rerun the stochastic, spatially structured population viability analysis described by Burkhart & Slooten

5 /.. - ) > IL.f 336 New Zealand Journal of Marine and Freshwater Research, 2006, Vol. 40 (V 1 V». ;^/ /- J 0 2 A N vy v V Vi " x" e S WTA *? (~~~---- S a 8 10najdcalmile5(1S5kn] i / Lata j V i: IIJÍLIL 1J l F3J f, f -r * rri' j^i A.. U»il Hali_< : Tu A - V" f It b ^ v $&.=U-.,V. F Sxn f v " / *^ 'Hi ^3 PENir* sum /& Tu 3ÉF<3 UMIV.Ï-Y V " rídl < T 3 y N s ; V, M 1 F-- f > > u! <.. "4S ** n -Vr d > v y i : V > < y/ K V s 1 % V '/ >r t " (A MV J ) L 'S / ^ / Fig. 2 Layout of potential transect lines around Banks Peninsula, New Zealand. Basemap modified from Land Information New Zealand chart NZ 64 (LINZ 2003, used with permission). The dashed line 4 nautical miles offshore represents the extent of the Banks Peninsula Marine Mammal Sanctuary. The 12 nautical mile territorial limit is also shown (+++). Spot soundings and depth contours are in metres. * ir, :... es / /...- : I f IS 1 ZL --_X r ^ ^ - ^. vmtf**""j^- ^-- _^^P-*^ isca.* 10 (2003) for the Banks Peninsula population. This is a logistic, density-dependent population model (Schaefer 1954) of the form: N t+1 = N t x. -1) (1 - N t /K)] - Nt (1) where N t =population size at time t, X^^ = maximum annual population growth rate, K=carrying capacity, and C t = the proportion of the population killed by entanglement in gill nets in year t. This model was used to project the population forward 100 years from the most recent population size estimate (1997/98; DuFresne et al. 2001; Dawson et al. 2004). Population size in 1970 was used instead of carrying capacity (K). This is approximately when the introduction of inexpensive nylon nets facilitated a large increase in the gill-net fishery in New Zealand. Population size in 1970 was estimated using the method of back-calculation described in Smith & Polacheck (1979; see Barlow & Hanan 1995 for a similar application). We made these calculations in an Excel spreadsheet. The spreadsheet started with population size in 1970, and used Equation 1 to calculate forward until the present, and continuing into a future population projection. Excel's goal seek function was then used to estimate the population size in 1970 that would result in the appropriate population size for the year in which the last population survey was carried out. The model was used to estimate the risk of decline for Hector's dolphins in fisheries areas 20 and 22 (Fig. 1). Data on fishing effort was gathered at the level of these relatively large areas. The Ministry of Fisheries provided data on commercial fishing effort in the form of catch effort and landing return (CELR) summaries (Burkhart & Slooten 2003).

6 Slooten et al. Hector's dolphin offshore distribution 337 The fishing industry carries out additional logbook programmes, which provide information on more precise fishing locations within these statistical areas (Starr & Langley 2000). However, these data are available for a relatively small number of fishers and fishing years. Discussions with the Sea Food Industry Council (Colin Sutton and Adam Langley pers. comm.) helped us exclude fishing effort that was outside dolphin habitat (e.g., gill-net fishing in Lake Ellesmere, in area 22) on the basis of these logbooks and other anecdotal information. For the purposes of this analysis we have kept our calculations very simple, using fishing effort for areas 20 and 22 combined and the total number of dolphins exposed to gill-net fishing (outside the sanctuary). The maximum annual population growth rate (λmax.) f r Hector's dolphin has been estimated by Slooten & Lad (1991) using two different methods. They used Reilly & Barlow's (1986) model, with the same survival rate estimate for all non-calf age classes and a lower survival rate for calves, resulting in λmax. estimates of and They also used a survivorship curve model which resulted in λm estimates of (using a survivorship schedule based on data from northern fur seals) and (based on survival rates of adult human females at the turn of the century). These values represent the maximum population growth rates feasible for Hector's dolphins, using data on reproduction from field studies of Hector's dolphin and optimistic survival rate schedules for other species. It is not possible to estimate survival rates for non-impacted Hector's dolphin populations as all populations are subject to some level of bycatch in gill-net and trawl fisheries (Dawson et al. 2001). Martien et al. (1999) and Burkhart & Slooten (2003) provide more detailed information on input data and model structure and describe a deterministic and stochastic version of the model, respectively. The sanctuary comprises parts of fisheries management areas 20 and 22 (Fig. 1), the areas for which fishing effort data were available. Using data from a boat-based line-transect survey (see DuFresne et al. 2001, Dawson et al for data and analysis methods), we calculated estimates of Hector's dolphin population size in a strip 4 nautical miles from the coast in both fisheries areas 20 and 22 in summer. Population size estimates were 228 (SE 90.9) for area 20 and 1187 (SE 247.4) for area 22.The proportion of the population found outside 4 nautical miles in the aerial surveys was applied to these estimates to obtain estimates of total population size to 15 nautical miles offshore. To determine the average exposure of dolphins to gill nets we took into account the relative amount of fishing in summer and winter. Starr & Langley (2000) estimated that 71% of the gill-net fishing effort (in total metres of net used) occurs in spring and summer (October to March) and the remaining 29% in autumn and winter (April to September). The proportion of dolphins exposed to gill-net fisheries in any given year was therefore estimated as a weighted average, multiplying the proportion of dolphins found outside the sanctuary and the proportion of fishing effort for each season. We also compared the number of Hector's dolphins estimated to be caught currently with the maximum level of human impact that would be allowed in the United States (see Wade 1998): M = (Nmin.) (0.5 Rmax.) (F r ) (2) where M=maximum allowable human impact (called PBR, or potential biological removal, in the United States and MALFIRM, or maximum allowable level of fishing related mortality, in New Zealand), N min. = lower 20th percentile of the population size estimate, R max. = maximum theoretical or estimated net productivity rate of the population, assumed to be reached at a small population size, and F r = recovery factor between 0.1 and 1, recommended to be set at 0.1 for endangered species (Barlow et al. 1995; Wade 1998). In this model, the population dynamics of marine mammals are assumed to follow a logistic curve. Therefore, the maximum number of individuals are added to the population each year when the productivity per individual is 0.5 R max. and population size is at MNPL. An extensive simulation study showed that the specifications above result in a 95% probability that populations starting at MNPL stay at or above that level after 20 years and populations starting at 30% of K recover to at least MNPL after 100 years (Wade 1998). The Banks Peninsula Hector's dolphin population is estimated to have declined to 35% of its population size in 1970 (see Burkhart & Slooten 2003). A recovery factor of 0.1 would ensure that continued bycatch results in only minor delays in population recovery. The main reason for including F r is to account for potential bias in the estimation of population size, productivity, and fisheries mortality (Wade 1998). Equation 2 uses 0.5 R max., as the goal of the management procedure is to keep populations at or above the level at which this level of productivity (the maximum net productivity) is reached (Wade 1998). The default value used for R max. is 0.04, as

7 338 New Zealand Journal of Marine and Freshwater Research, 2006, Vol. 40 data on whales and dolphin populations indicates this is likely to be the maximum for these animals (Perrin et al. 1984; Reilly & Barlow 1986; Wade 1988; Barlow et al. 1995). We used this value in our calculations, although this is an optimistic estimate for Hector's dolphins. Studies of survival rates, reproductive biology and population dynamics of Hector's dolphins indicate that an R max. as high as 0.04 would require a very optimistic survival schedule similar to that of humans (Slooten & Lad 1991; Martien et al. 1999). Given that we used an R max. of 0.04 in our calculations, the actual risk to the Banks Peninsula Hector's dolphin population is likely to be somewhat greater than indicated. RESULTS A similar number of Hector's dolphins were sighted in summer and winter (Table 1). Hector's dolphin sightings were strongly clustered in shallow, inshore waters in summer (Fig. 3) and more evenly distributed throughout the survey area in winter (Fig. 4). In summer, almost 78.6% of the Hector's dolphin groups sighted were inside the 4 nautical mile sanctuary boundary (Fig. 5A). In contrast, 35.1% of the Hector's dolphin groups sighted in winter were inside the 4 nautical mile sanctuary boundary (Fig. 5B) and dolphin densities stayed relatively high out to the 15 nautical mile limit of the survey area. Thus, Hector's dolphins may range beyond the 15 nautical mile extent of our survey at Banks Peninsula in winter, and the proportion of the population protected by the sanctuary in winter could be somewhat lower than the estimated 35.1%. Using this new information on offshore distribution in the population model increased the estimated risk of population decline. For example, with a population growth rate of per year, the Hector's dolphin population in fisheries areas Table 1 Summary of sighting data for summer and winter aerial surveys around Banks Peninsula, New Zealand. Feb 2002 Jun 2002 No. groups Mean CV (%) of sighted group size group size and 22 (Fig. 1) would be expected to have a probability of decline of 44% (see fig. 6 in Burkhart & Slooten 2003). Burkhart & Slooten's (2003) analysis included potential gill-net entanglement north and south of the sanctuary boundaries (Sumner to the Rakaia River) but assumed that the dolphin population between Sumner and the Rakaia River was fully protected by the sanctuary. Taking into account the proportion of dolphins offshore of the sanctuary boundaries increased the risk of population decline to 62%. Likewise, for a population with a growth rate of 1.023, the risk of population decline increased from 18% to 35% when the proportion of dolphins offshore of the sanctuary boundaries was taken into account. We calculated bycatch limits based on the United States PBR model for the entire dolphin population in fisheries areas 20 and 22 (i.e., if the sanctuary is ignored) and for the dolphins in fisheries areas 20 and 22 that are outside the sanctuary. In both instances, we used R max. = 0.04 and F r = 0.1 (the default R max. value for cetaceans and the default F r value for endangered species, see Materials and Methods). N min., the lower 20th percentile of the population size estimate was 1419 for the entire dolphin population in fisheries areas 20 and 22, resulting in a bycatch limit of 2.8 dolphins per year. For the dolphins in fisheries areas 20 and 22 that are outside the sanctuary (north, south, or offshore) the situation is more complex as this proportion of the population varies owing to the seasonal changes in dolphin distribution described above. N min. for the dolphins found outside the sanctuary boundaries was 602 during summer and 967 during winter, resulting in a PBR of 1.2 for summer and 1.9 dolphins per year for winter. The average of these two values (1.6 dolphins per year) was used as a management target for our calculations of how much the sanctuary would need to be extended to achieve a sustainable level of bycatch for the dolphins presently outside the sanctuary. Current bycatch could be reduced to this level by enlarging the sanctuary or reducing fishing effort, or both. The number of Hector's dolphin entanglements in the commercial gill-net fishery was estimated in an observer programme during the 1997/98 fishing season. Observer coverage in subsequent years has been too low to allow the level of bycatch to be estimated (Dawson & Slooten 2005). In the 1997/98 fishing season, an estimated 17 dolphins were killed in the commercial gill-net fishery (Baird & Bradford 2000; Starr 2000). Given this catch rate, and current levels of gill-net fishing effort, the number of

8 Slooten et al. Hector's dolphin offshore distribution 339 Fig. 3 Survey lines and sightings in summer (February 2002). Numbers in parentheses indicate a line which was replicated. dolphins exposed would need to be reduced to 412 to achieve a bycatch limit of 2.8 dolphins, or 135 to achieve a bycatch limit of 1.6 dolphins per year in the commercial gill-net fishery alone. This could not be achieved solely by extending the offshore boundary of the sanctuary. A 15 nautical mile offshore boundary would still leave 674 dolphins in fisheries areas 20 and 22 exposed to gill-net fisheries immediately north and south of the sanctuary. To reduce this to 412 would require an additional 39% of both north and south populations to be protected. This would require extending the northern boundary to approximately Leithfield Beach and the southern boundary to the Rangitata River (Fig. 1). To reduce the number of dolphins outside the sanctuary area to 135 would require extending the sanctuary to approximately Black Birch Creek in the north and Wainono Lagoon in the south. In both instances the offshore boundary would need to be extended to 15 nautical miles (Fig. 1). DISCUSSION This survey extended previous boat surveys which went offshore to 5 nautical miles (Dawson & Slooten 1988) and 10 nautical miles (Dawson et al. 2004). In summer, dolphin sightings were strongly clustered in shallow, inshore waters. This preference for inshore waters was much less pronounced in winter, when dolphins were more evenly distributed throughout the surveyed area. This matches the results from other surveys carried out in the Banks Peninsula area (e.g., Dawson & Slooten 1988; Bräger 1998). Hector's dolphins are found closer to the coastline in summer, and they seldom enter larger harbours and bays outside the summer months (Dawson 1991). Similar numbers of sightings were made in the summer and winter aerial surveys, and the same individual dolphins have been sighted in the same areas in summer and winter (Bräger et al. 2002). There is no evidence that dolphins leave the

9 340 New Zealand Journal of Marine and Freshwater Research, 2006, Vol. 40 Fig. 4 Survey lines and sightings in winter (June 2002). Banks Peninsula area during winter; instead their distribution becomes more dispersed. The much stronger preference for inshore waters in summer is likely to be related to prey distribution patterns. Several of their prey species come closer to shore in summer to spawn (L. Paul pers. comm.). It is also possible that Hector's dolphins seek more sheltered waters during spring and summer when calves are born or that they come inshore for other social reasons like mating. However, the west coast of the South Island is a stronghold for Hector's dolphins (Slooten et al. 2004), and lacks similar shelter. A more dispersed distribution in winter is also observed in Hector's dolphin populations off the west coast of the North Island (Slooten et al. 2005, 2006) and the west and south coasts of the South Island (Bejder et al. 1999; Rayment et al. 2003; Slooten et al. 2004). The maximum offshore distance of dolphin sightings does not appear to change seasonally but differs markedly among the three areas studied. In aerial surveys of similar design, the furthest offshore sightings were 4 nautical miles off the North Island west coast, 6 nautical miles off the South Island west coast, and at least 15 nautical miles off Banks Peninsula (Rayment et al. 2003; Slooten et al. 2005, 2006). The common denominator appears to be water depth, with all sightings made in less than 90 m of water. The broad offshore distribution at Banks Peninsula appears to result from the shallowly shelving bathymetry, and may help to explain why the seasonal difference in offshore distribution is strongest at Banks Peninsula. This seasonal difference in offshore distribution undoubtedly varies from year to year. The survey described in this paper was the first in a series of three summer and three winter surveys at Banks Peninsula. This series of surveys is still incomplete. However, it is already clear that despite the year-toyear variability, the data show a consistent pattern with a high proportion of dolphins protected inside the sanctuary in summer (mean 82%, SD 9.7) and

10 Slooten et al. Hector's dolphin offshore distribution n 14 en 1 2 I 10 n n n n o n Distance offshore (nautical mües) inside the sanctuary. Reducing bycatch to the levels recommended by the NMFS model would involve extending the offshore boundary to 15 nautical miles in addition to substantial extensions of the north and south boundaries. In practice, the sanctuary would need to encompass most of fisheries areas 20 and 22. An integrated approach, including extensions to the sanctuary boundaries (north, south, and offshore) as well as reductions in overall fishing effort in the area would be the most effective way of reducing bycatch to sustainable levels. 10 to o ö O) IM 'S 4- S 2- n nn n DDn n Distance offshore (nautical miles) Fig. 5 Offshore distribution of Hector's dolphin Cephalorhynchus hectori sightings on transects at Banks Peninsula in: A, summer (February 2002); B, winter (June 2002). a much lower proportion in winter (mean 39%, SD 5.6; Rayment et al. 2006). The implications for the Banks Peninsula Marine Mammal Sanctuary are that the proportion of the population that is protected is much smaller than envisaged, about 79% in summer and 35% in winter. Our most recent population viability analysis (Burkhart & Slooten 2003) assumed that the offshore boundary enclosed the entire sanctuary population. In other words, we assumed that the sanctuary was 100% effective in protecting from bycatch all dolphins found within its north-south boundaries. At that time, we did not have an estimate of the proportion of the population found inside the 4 nautical mile offshore boundary. Correcting this to 78.6% in summer and 35.1% in winter increases the risk of population decline from c. 40% to more than 60%. The risk of continued population decline can be reduced by extending the sanctuary. Decisions on new sanctuary boundaries would also need to take into account dolphins caught in gill nets used by amateur fishers, bycatch in trawl nets, and other human impacts including boat strikes (Stone & Yoshinaga 2000). In addition, compliance with sanctuary regulations would need to be taken into account. We are not aware of any research on the level of compliance. However, commercial gill nets are frequently seen on the sanctuary boundaries and illegal amateur gill nets are regularly found ACKNOWLEDGMENTS We thank the New Zealand Whale and Dolphin Trust for funding these surveys, Timor Kabar and Vaughn Richardson from the Canterbury Aero Club for piloting the plane, and Hendrik Nollens for being an observer. Discussions with staff from Department of Conservation and World Wide Fund for Nature helped us refine the survey design and analysis methods, to ensure that the results were useful for conservation management. We are grateful to Land Information New Zealand for allowing us to use their electronic charts. Comments from three anonymous referees helped us improve the manuscript. REFERENCES Baird SJ, Bradford E Estimation of Hector's dolphin bycatch from inshore fisheries, fishing year. Published Client Report on Contract 3024, Funded by Conservation Services Levy. Wellington, Department of Conservation. 20 p. ( [accessed 15 October 2004]) Baker AN, Smith ANH, Pichler FB Geographical variation in Hector's dolphin: recognition of new subspecies of Cephalorhynchus hectori. Journal of the Royal Society of New Zealand 32: Barlow J, Hanan D An assessment of the status of harbor porpoise in Central California. In: Bjorge A, Donovan GP ed. Biology of Phocoenids: a collection of papers. Cambridge, International Whaling Commission. Pp Barlow J, Swartz SL, Eagle TC, Wade PR U.S. marine mammal stock assessments: guidelines for preparation, background, and a summary of the 1995 assessments. NOAA Technical Memorandum NMFS-OPR (Available from US National Marine Fisheries Service.) Bejder L, Dawson SM, Harraway J Responses of Hector's dolphins to boats and swimmers in Porpoise Bay, New Zealand. Marine Mammal Science 15:

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