The Reproductive Patterns of Salmonids: Factors Affecting Male Reproductive Success

The Reproductive Patterns of Salmonids: Factors Affecting Male Reproductive Success

Factors affecting male reproductive success Fertilization success Female choice Male competition Attributes of the female Fecundity Redd quality, etc.

Female choice Female salmon establish redds and therefore have a limited choice of mates (unlike many other mating systems). Female choice is primarily exerted via delay in breeding when courted by small males. Excessive delay may come at a cost to nest guarding.

Intra-sexual competition Display and combat for position near the female at the moment of egg deposition (Alpha male) Alternative approach: accept a secondary satellite position and try to fertilize a fraction of the eggs by timely approach to the female (sneaking)

The absolute sex ratio is normally close to 50:50 in salmon but it is the Operational Sex Ratio ( OSR ) that determines the levels of competition, hence variation in male reproductive success. OSR = # of sexually active males # of sexually active females Because females deposit their eggs soon after arriving on the spawning grounds, the OSR becomes skewed, increasing male competition.

Males can easily monopolize access to females when the OSR is nearly even.

Arrival patterns and the operational sex ratio Number of salmon 500 400 300 200 100 0 arrival live fish ripe females Lifespan = 14 7 d Spawn in 4 d 0 6 12 18 24 Day of the spawning season A

Arrival patterns and the OSR: male breeding opportunity = number of days alive * inverse of the OSR each day Male breeding opportunities 8 7 6 5 4 3 2 1 0 1 6 11 16 21 Day of the spawning season B male breeding opportunities OSR 16 12 8 4 0 Operational sex ratio

Males arrive before females because they lose more opportunities to breed by being late than early % males arrived 100 50 Hansen Creek 2 day moving average 0 16-Jul 26-Jul 5-Aug 15-Aug

OSR determines the levels of competition, and size and shape affect dominance (and female choice), but do the dominant males Steven Schroder fertilize all the eggs?

Dominant males fertilize most but not all eggs % of eggs fertilized 100 75 50 25 0 1 2 3 4 5 6 Dominant male chum salmon Schroder 1981

Jacks are sexually mature males that have spent fewer years at sea than females of the population (e.g., 1 rather than 2 or 3 winters in sockeye; only a summer rather than a year plus a summer in coho.

Mature male UW hatchery chinook salmon Age 1 Age 2 Age 3 Age 4

Males may also mature as parr, without going to sea, in some species.

As males get older and larger, their gonads are larger on an absolute basis but smaller as a proportion of their weight than those of smaller males. GSI = gonad weight as a % of body weight. age fork length body weight gonad weight GSI 1 23.9 185 26 14.1 2 47.8 1355 130 9.6 3 68.5 3810 284 7.5 4 86.4 7517 425 5.7 data: University of Washington hatchery

Mart Gross hypothesized that jacks are not aberrant or inferior but rather are an alternative life history pathway with equal reproductive success. Fitness = Probability of surviving to maturity times the reproductive success of survivors

Marine survival: Breeding lifespan (d): Jacks = 0.13 Hooknoses = 0.06 Jacks = 8.4 Hooknoses = 12.7 Relative proximity to spawning female: Jacks = 0.66 Hooknoses = 1.0 Fitness: Jacks = 0.13 * 8.4 * 0.66 = 0.72 Hooknoses = 0.06 * 12.7 * 1.0 = 0.76 = 0.95

Equal volumes of milt from different males do not fertilize equal numbers of eggs. % of eggs fertilized by each male Female Male A Male B Male C Male D 1 2 18 13 67 2 4 12 38 46 Gharrett and Shirley 1985

Carla Stehr

Balancing natural and sexual selection Wood River Lakes System ALASKA Iliamna Lake Bristol Bay

Populations vary in age at maturity and length

Habitat and life history of creek and river sockeye salmon populations in the Wood River system % marine age 3 90 80 70 60 50 40 30 20 10 0 females males 1.0 10.0 100.0 spawning site width (m)

Why be large? Large females make larger and more numerous eggs, and can bury them deeper than smaller females. Large males can dominate smaller ones for access to females, and are favored by females as mates.

Why not be large? Large size may: 1) necessitate a longer (hence riskier) stay at sea, or more aggressive (hence riskier) foraging there 2) limit access to shallow spawning sites. 3) increase vulnerability to predators

Hansen Creek sockeye salmon

Copper River sockeye salmon

Knutson Bay sockeye salmon

Relationship between spawning site depth and body depth (standardized for length) for Wood River and Iliamna Lake sockeye male body depth (mm) 200 175 150 125 100 R 2 = 0.83 0 100 200 300 spawning site depth (cm)

Observation: Pacific salmon vary greatly in size and shape at maturity, among and within populations. Hypothesis: This variation results from conflicting patterns of natural and sexual selection.

Role of sexual selection in body size and shape of male sockeye salmon General approach: Catch mature salmon, measure them, tag them, observe them during their reproductive lives and record longevity and social dominance

Hansen Creek

Woody Island, Iliamna Lake Beach spawning sockeye show extreme sexual dimorphism

Snorkeling in Iliamna Lake

Results: Longer males were more often dominant than smaller males courting females, probably through a combination of male competition and female choice. However, males that were deep-bodied for their length were also more successful. Thus sexual selection seemed to favor both size and shape.

Male Hansen Creek sockeye salmon

Size frequency and age of Hansen Creek sockeye, and % killed by bears 5000 100 4000 3000 males count % killed 2 3 80 60 2000 1000 0 226-250 251-275 1 276-300 301-325 326-350 351-375 376-400 401-425 426-450 451-475 476-500 501-525 526-550 551-575 > 575 40 20 0

Bears kill a higher percentage of the salmon in small streams than larger streams

Conclusions Sexual selection tends to favor large size through gonad production and other benefits in females, and access to females in males Natural selection counters this by culling fish that spent too long at sea and by features of the breeding environment that oppose large size The age, size and shape of populations reflects the different balances between these processes

Male Reproductive Success Number of eggs fertilized Access to females (and female fecundity) Survival of embryos Redd site quality Redd disturbance Environmental factors Female choice: male size, shape, color Tactics: delay and attack Male-male competition: A. Extrinsic factors: operational sex ratio sneakers and fighters B. Intrinsic factors: body size and shape prior residency aggression sperm quality