Spawning Sockeye Salmon Research Articles

Unaccounted mortality in salmon fisheries: non‐retention in gillnets and effects on estimates of spawners

Summary 1. Effective and sustainable natural resource management is enhanced when the consequences of exploitative practices are fully understood and acknowledged. Commercial fisheries devote considerable resources to maximize the harvest of target species and minimize interference with non‐target stocks. Appropriately, bycatch and discard of non‐target stocks are recognized as critical economic and conservation concerns. Few studies, however, have examined non‐retention mortality in target stocks. Non‐retention, where fish are engaged by fishing gear but not landed, is rarely quantified and the effects on stocks are unknown. Mortality due to non‐retention may have important effects on the dynamics of exploited populations. 2. We surveyed spawning populations of sockeye salmon Oncorhynchus nerka that had traversed commercial fisheries in Bristol Bay, Alaska, to estimate the incidence of non‐retention in gillnets and the severity of injuries associated with entanglement. To better understand how gillnet injury affects spawning success, we tagged and monitored stream‐spawning fish and applied a maximum likelihood model to mark–recapture data. 3. A substantial portion (11–29%) of spawning sockeye salmon exhibited clear signs of past entanglement with commercial gillnets. Survival among such fish was significantly reduced. More than half of the fish that reach natal spawning grounds with fishery‐related injuries fail to reproduce. This suggests that estimates of spawning stocks are inflated by 5–15% at minimum. 4. Synthesis and applications. Our analyses indicate that non‐retention in gillnet fisheries is an important and under‐appreciated consequence of the exploitation of salmon. Stock estimates for exploited populations that do not account for non‐retention mortality overestimate the number of reproductively viable fish. Unaccounted mortality and interannual variation in the magnitude of this mortality may prevent accurate estimates of viable spawners, confound our understanding of the relationship between stock size and recruitment, impede optimal management and obscure the ecosystem impacts of migratory stocks in coastal watersheds. Given the magnitude of non‐retention in this fishery, explicit consideration of non‐retention mortality may be warranted across a wide range of exploited populations.

Field trials of a new physiological data logger in active fishes

We have designed and constructed implantable data loggers that measure heart rate (HR) and body temperature of free‐ranging animals. We present data from the first field trials of these loggers in penned southern bluefin tuna (Thunnus maccoyii) under aquacultural conditions, and in spawning sockeye salmon (Oncorhynchus nerka) under natural conditions. Bluefin tuna (~ 15.2kg) maintained visceral temperature (VT) between 0.4 – 2.6°C above ambient water temperature (18.5°C) during non‐digesting periods, while HR ranged from 33 – 74 beats/min. VT rose to 1.4 – 6.5°C above ambient at the peak of the digestive period, and the magnitude of this increase was dependent on ration size. HR followed a similar trend during the digestive period, reaching levels of 52 – 117 beats/min and implying that the postprandial increase in oxygen consumption is assisted largely by an increase in HR. The duration of digestion ranged from 10.7 to 30.9 h depending on ration size.Sockeye salmon were implanted with data loggers in October 2007 when they had migrated from the ocean to their natal stream to spawn and subsequently die. Data analyses are ongoing, but it has been confirmed that salmon carrying data loggers do not have a reduced lifetime on the spawning ground, implanted salmon display normal courting and mating behaviours, and implanted salmon completed their lifecycle normally by spawning before death.

Variable effects of marine‐derived nutrients on algal production in salmon nursery lakes of Alaska during the past 300 years

We measured historical changes in sedimentary δ15N and fossil pigments in four lakes with anadromous semelparous salmon and two reference lakes to quantify the degree to which the flux of marine‐derived nutrients (MDNs as N isotopes) regulate algal production (as pigments). During the past 300 yr, production of the predominant algae (diatoms) was positively correlated (r = 0.42–0.93, p < 0.02) with sedimentary δ15N in nursery lakes of sockeye salmon (Oncorhynchus nerka) but was inversely correlated with sedimentary δ15N (r = 20.71 to 20.73, p < 0.0001) in reference lakes that lacked migratory fishes. Overall, the pigment‐δ15N correlation during the 20th century was strongly correlated with both mean densities of spawning sockeye salmon during 1956–2000 (r = 0.97, p < 0.002) and the fraction of total ecosystem N derived from salmon during 1900‐2000 (r = 0.98, p < 0.001). Together these patterns suggest that the sign of the δ15N‐pigment correlation can be used to distinguish among lakes or periods of time in which algal production is regulated mainly by MDN influx (positive correlation) or other factors (negative correlation). Tests of this hypothesis revealed that the degree to which MDNs regulated algal production in nursery lakes varied greatly since 1700, with significant periods of weak control even in lakes with abundant salmon. Further, when considered at the landscape scale, the importance of MDNs to individual lakes varied substantially through time and in space, with little evidence of synchrony among sites or catchments.

SEDIMENT DISPERSION IN SALMON SPAWNING STREAMS: THE INFLUENCE OF FLOODS AND SALMON REDD CONSTRUCTION1

ABSTRACT: Magnetically tagged particles were used to investigate the effects of sockeye salmon (Oncorhynchus nerka) and floods on the dispersion of coarse bed material in the Stuart‐Takla region, British Columbia, Canada. The dominant annual sediment transporting event in the channels is the snowmelt flood events, with lesser activity usually accomplished during summer floods. Annually in August, the channel bed material is reworked by the Early Stuart sockeye salmon spawning, as the fish excavate the streambed to deposit and bury their eggs. These nesting cavities are called redds. Results from 67 tracer recovery experiments over five years were highly variable, subject to the magnitude of floods and the returning population of salmon. Overall, the depositional pattern from nival flood events usually demonstrated a high degree of clast mobilization, long travel distances (up to 150 m), and mean depths of burial up to 18 cm. Summer flood events showed somewhat lower rates of mobilization, distances of travel, and depths of burial. Although the fish did not move the tracers very far, their effect on the bed was generally quite pervasive ‐ up to 100 percent of the clasts were mobilized, and the depth of burial was considerable (mean burial depths up to 14 cm). The amount of vertical mixing of sediment by salmon was often on the same order of magnitude as flood events. The significant vertical mixing of sediments by the fish has important implications for the mobility of sediment in the stream. Since any armoring layer formed during high flows throughout the year is subject to the bioturbation of salmon, this suggests that the transport threshold in the creeks remains relatively low. Salmon likely play an integral role in the sediment transport dynamics and annual sediment budget of the lower reaches of these creeks.

Disturbance of freshwater habitats by anadromous salmon in Alaska.

High densities of habitat modifiers can dramatically alter the structure of ecosystems. Whereas spawning sockeye salmon ( Oncorhynchus nerka) dig nests that cover over 2 m(2) and are at least 20 cm deep, and can spawn at high densities, relatively little attention has been devoted to investigating the impacts of this disturbance. We hypothesized that this temporally and spatially predictable bioturbation has large impacts on the coastal aquatic habitats used by sockeye. We experimentally investigated the impacts of disturbance caused by spawning sockeye in two streams and two lakes in Alaska by excluding salmon from 2.25 m(2) plots where they traditionally spawn. We sampled exclusions and control plots before, during, and after spawning. During sockeye spawning, fine sediment accumulated in areas where sockeye were excluded from spawning. In addition, sockeye spawning significantly decreased algal biomass by 80% compared to exclusion plots. We found mixed effects of spawning on the invertebrate assemblage. Tricladida and Chironomidae densities increased by 3x in exclusion plots relative to control plots in one creek site. However, for most taxa and sites, invertebrate densities declined substantially as spawning progressed, regardless of experimental treatment. Habitat modification by spawning salmon alters both community organization and ecosystem processes.

Effects of the abundance of spawning sockeye salmon (Oncorhynchus nerka) on nutrients and algal biomass in forested streams

We used natural variation in sockeye salmon (Oncorhynchus nerka) spawner biomass among sites and years in three undisturbed, forested watersheds in interior British Columbia to test the hypotheses that salmon were a major source of particulate organic matter inputs to the streams and that carcass biomass determined stream-water nutrient concentrations and epilithic algal production. Sockeye carcasses were retained at the spawning sites, primarily (75–80%) by large woody debris (LWD) or pools formed by LWD. The abundance and distribution of sockeye salmon determined stream-water nutrient concentrations and epilithic chlorophyll a concentrations during late summer and early fall when most primary production occurred in the oligotrophic streams. Periphyton accrual rates were elevated at sites with high salmon biomass. Peak chlorophyll a concentration increased with increasing carcass biomass per unit discharge above a threshold value to reach maxima 10-fold greater than ambient levels. Epilithic algae were dominated by a few common, large diatom taxa. Salmon carcasses were the dominant source of particulate organic carbon in low gradient stream reaches. Nutrient budget modeling indicated that most of the salmon-origin nutrients were exported from the spawning streams or removed to the terrestrial ecosystem; diffuse impacts may extend over a much larger area than simply the sites used for spawning.

Divergence of Mitochondrial DNA in Populations of Sockeye Salmon Oncorhynchus nerka Walbaum from Azabach'e Lake (Kamchatka)

Variability of three PCR-amplified mtDNA regions was examined in five populations of sockeye salmon from Azabach'e Lake. Eighteen haplotypes were detected in 144 fish. Significant differences were found between seasonal races of sockeye salmon spawning in the lake. The short time of independent divergence between the seasonal races indicates that these races formed independently in each spawning region. No difference in mtDNA between lake samples of early sockeye salmon (subisolates) was revealed, which confirms the existence of gene flow between them. A high level of differences between the sockeye salmon spawning in the lake and spawning in the tributaries of the lake, the Bushuev and Lotnaya rivers, suggests that there were no migration between them during many generations and that the nature of spawning grounds (lake or river) is essential for within species differentiation in this species.

Population Structure of Sockeye Salmon of the Central Coast of British Columbia: Implications for Recovery Planning

Abstract The populations of sockeye salmon Oncorhynchus nerka of the south-central coast of British Columbia are in decline. To assist in recovery planning, we determined the population structure of sockeye salmon in the region by assaying the genetic variability of 10 microsatellite DNA loci in samples of sockeye salmon from 22 sites associated with 15 rearing lakes. Samples of sockeye salmon from the watersheds with the largest historical runs in the region were studied. Special emphasis was given to investigating genetic divergence among sockeye salmon spawning in seven rivers of the Owikeno Lake watershed, which once supported the largest sockeye salmon run in south-central British Columbia. Across the region, a mosaic of genetic divergence was evident. Reproductive isolation among watersheds was pronounced in all but one case, making transfer of fish between watersheds inadvisable. Genetic stock identification simulations demonstrated that fish from different watersheds could be accurately distinguis...

Evidence for Fine-Scale Natal Homing Among Island Beach Spawning Sockeye Salmon, Oncorhynchus nerka

Salmonid fishes aggregate for breeding at spatially defined, suitable habitats. These aggregations may evolve into discrete populations when precise natal homing leads to reproductive isolation, and local regimes of selection lead to adaptation. Population structure is often defined by persistent differences in selectively neutral genetic markers and in mean values of morphological and life-history traits between locations. This approach is limited by the spatial scale at which traits diverge; low levels of reproductively successful straying, combined with similar selective pressures on life-history traits resulting from similar habitat features and environmental conditions, can significantly reduce the power of these discriminatory methods. We compared data on three life-history traits and polymorphism of DNA microsatellites for evidence of population subdivision among sockeye salmon spawning on spatially discrete but physically similar beaches on islands in Iliamna Lake, Alaska. We found small but significant differences in average body length, body depth and age composition between sites as well as significant interactions between site and year. These interactions, reflecting random variation in growth or recruitment among sites, are a powerful tool for discriminating populations with similar mean trait values. These results suggest fine-scale homing to natal sites, but the microsatellite data revealed no evidence of restricted gene flow among sites. There seems to be enough straying among the populations to prevent differentiation at neutral traits but enough homing for them to be functionally distinct.

Energy expenditures during reproduction by sockeye salmon (Oncorhynchus nerka)

From 1994 to 1996, behaviour and energy use by sockeye salmon spawning in Gluskie Creek, British Columbia, was investigated by a combination of behavioural observation and electromyogram (EMG) telemetry. The spawning life was 7.6 days for males and 10.6 days for females. Both sexes held in pools for a few days, with an average cost of 7.9 kcal/day, before starting to spawn. During spawning, dominant males performed 6 behavioural acts per 10 min, charging being the most frequent followed by quivering, chasing, digging, lateral and posture displays. Spawning females performed 4.4 behavioural acts per 10 min, digging being the most frequent followed by charges and chases. Following egg deposition, females entered a nest guarding phase and, in the latter stages of spawning, males adopted subordinate behaviour. Guarding females and subordinate males performed 1.1 and 1.6 behaviours per 10 min respectively. Lateral and posture displays by males lasted 6.3 s and 11 s respectively but all other behaviours lasted less than 2 seconds. Holding behaviour and posture displays required the most energy in males and holding and digging in females. Dominant males and spawning females expended 23.9 kcal/day whereas guarding females and subordinate males expended 11.0 kcal/day. Frequency of behaviours during active spawning was similar to other populations. Length of spawning life was shorter, however, and total energy expended was less than estimates for other populations based on body constituent analysis. Although EMG telemetry may underestimate total energy expenditure, other recent measures of gross energy expenditure by Stuart River sockeye also indicate that energy available for reproduction was low in this population due to declining average size and low energy reserves in fish arriving at the spawning grounds.

Selection on Morphology of Spawning Wild Sockeye Salmon by a Gill-Net Fishery

Abstract Human activities can cause artificial selection in wild animals. To examine the effects of gill-net selectivity on locally differentiated populations of sockeye salmon Oncorhynchus nerka in Bristol Bay, Alaska, we completed a three-part study: (1) We showed differentiation in the body form of mature sockeye salmon spawning in beach and stream habitats that were separated by less than 300 m. (2) Because gill-net selection acts directly on the girth of immature sockeye salmon, we correlated girth at capture with the morphological characters distinguishing locally differentiated populations on the spawning grounds. By tagging individual fish and measuring them both when immature and when mature, we found morphology at maturity to be highly correlated with girth during immaturity. (3) Using selection regimes from the fishery catch and escapement data for 1994, we examined the effects of gill-net selectivity on populations of mature adults. We showed that although populations of mixed ocean age-classe...

Disturbance of Small-Stream Habitat by Spawning Sockeye Salmon in Alaska

Abstract We examined how the spatially and temporally predictable disturbance caused by a spawning run of native sockeye salmon Oncorhynchus nerka temporarily affected benthic and invertebrate drift communities of two small streams in southwestern Alaska. Surface gravel size temporarily increased as a consequence of this mechanical disturbance, and periphyton biomass decreased in one stream. The postspawning rise in periphyton biomass in Bear Creek (larger sockeye salmon run) likely reflects nutrient release by decomposing sockeye salmon. Total benthic invertebrate and larval chironomid density decreased during spawning in one stream, but most invertebrate taxa in both streams did not exhibit clear spawning-related impacts. In contrast, invertebrate drift almost always increased during daylight hours when sockeye salmon were present, but it was difficult to separate mechanical effects of sockeye salmon acting via redd construction versus swimming. Total daytime drift density during spawning was four times...

Selective predation by brown bears (Ursus arctos) foraging on spawning sockeye salmon (Oncorhynchus nerka)

Selective predation by and predation rates of brown bears (Ursus arctos) foraging on spawning sockeye salmon (Oncorhynchus nerka) in a small shallow creek in the Wood River lake system near Bristol Bay, Alaska, were quantified during 1986 and 1990–1992. Bears killed a high proportion of spawning salmon when few salmon entered the creek (92% of 505 fish) and a much smaller proportion when the spawning population reached a historical high (16% of 15 631 fish). Selective predation on salmon that differed in length, sex, and spawning condition was measured by tagging salmon at the mouth of the creek immediately prior to upstream migration and then recovering dead tagged fish during daily surveys of the entire creek. The relative frequencies of large, medium-sized, and small salmon killed by bears indicated that the risk of predation was more than 150% greater for large than for small salmon. A higher proportion of the male salmon population was killed and a greater proportion of male bodies were consumed than female salmon. Selectivity for male salmon increased as the spawning season progressed, possibly because male salmon weakened earlier and lived longer in a weakened state than female salmon. Male salmon were attacked mostly along the dorsal hump area, whereas female salmon tended to be attacked along the abdomen, where eggs could be exposed. Bears selectively killed female salmon prior to spawning during 1 of the 3 years, but only 6.1–7.8% of the female spawning populations were killed prior to spawning. These data support the hypothesis that selective predation by bears may influence the body morphology of spawning salmon.

Estimators of stream residence time of Pacific salmon (Oncorhynchus spp.) based on release-recapture data

The area-under-the-curve method is a widely used method for estimating salmon escapement. The method depends on obtaining an accurate estimate of stream residence time, or stream life. This paper develops two estimators of stream residence time based on release-recapture data: a nonparametric estimator and a parametric estimator. Monte Carlo simulations showed that with an adequate release size and number of sampling occasions, both estimators provide precise estimates of stream residence time. If there is significant right censoring, however, the parametric estimator is significantly less biased. If the data are too sparse, the parametric estimator performs poorly and often fails. The stream residence time of spawning sockeye salmon (Oncorhynchus nerka) in Iliamna Lake, Alaska, was estimated using the estimators developed here. Because the estimators also provide the variance of the estimates, the precision of the stream residence time estimate could be assessed, and we were able to test and reject the hypothesis that the stream residence time for females is equal to that of males. Both estimators are applicable to estimating the life expectancy of any fish or wildlife population with release-recapture techniques.

The role of chemoreception in salmon-egg predation by coastrange (Cottus aleuticus) and slimy (C. cognatus) sculpins in Iliamna Lake, Alaska

Egg predation by coastrange sculpins (Cottus aleuticus) and slimy sculpins (C. cognatus) may be a major factor affecting sockeye salmon (Oncorhynchus nerka) production in Iliamna Lake, Alaska. In this study, we examined the potential roles of visual and chemosensory cues in egg predation by sculpins. A field study tested whether sculpins were differentially attracted to minnow traps baited with eggs that were (i) visible with no odours present, (ii) visible with odours present, or (iii) not visible with odours present. Our results indicated that sculpins do not require visual cues for detecting salmon eggs. However, attraction to sockeye eggs did require chemical cues emanating from the eggs. To characterize the chemical attractants that emanate from salmon eggs, we tested whether sculpins were attracted to test odours in a two-choice maze. Test odours were prepared by soaking eggs in lake water. Sculpins preferred egg wash to lake water but demonstrated no attraction to ovarian fluid versus lake water, suggesting that the attractive substances are derived directly from egg material. These results are discussed in relation to the known sensory mechanisms involved in feeding by sculpins as well as to the ecological relationship between spawning sockeye salmon and sculpins.

Ecological relationship between freshwater sculpins (genus Cottus) and beach-spawning sockeye salmon (Oncorhynchus nerka) in Iliamna Lake, Alaska

The interaction between two sculpin species, Cottus cognatus and Cottus aleuticus, and island beach spawning sockeye salmon (Oncorhynchus nerka) was examined in Iliamna Lake, Alaska. We conclude that sculpins actively move to specific spawning beaches and that the initiation of their movements precedes the start of spawning. Sculpin predation on sockeye eggs is positively dependent on sculpin size and on the state of the eggs (fresh versus water hardened), with the largest sculpins able to consume nearly 50 fresh eggs at a single feeding and 130 over a 7-day period. The number of sculpins in sockeye nests is greatest at the beginning of the spawning run, lowest in the middle, and high again at the end, with peak numbers of over 100 sculpins per nest (1 m2). We discuss the results in terms of energy flow of marine-derived nutrients into an oligotrophic system and in terms of the coevolution of sockeye spawning behavior and the predatory behavior of sculpins.

Ecological relationship between freshwater sculpins (genus <i>Cottus</i>) and beach-spawning sockeye salmon (<i>Oncorhynchus nerka</i>) in Iliamna Lake, Alaska

The interaction between two sculpin species, Cottus cognatus and Cottus aleuticus, and island beach spawning sockeye salmon (Oncorhynchus nerka) was examined in Iliamna Lake, Alaska. We conclude that sculpins actively move to specific spawning beaches and that the initiation of their movements precedes the start of spawning. Sculpin predation on sockeye eggs is positively dependent on sculpin size and on the state of the eggs (fresh versus water hardened), with the largest sculpins able to consume nearly 50 fresh eggs at a single feeding and 130 over a 7-day period. The number of sculpins in sockeye nests is greatest at the beginning of the spawning run, lowest in the middle, and high again at the end, with peak numbers of over 100 sculpins per nest (1 m2). We discuss the results in terms of energy flow of marine-derived nutrients into an oligotrophic system and in terms of the coevolution of sockeye spawning behavior and the predatory behavior of sculpins.

Estimation of Surface and Intragravel Water Flow at Sockeye Salmon Spawning Beaches in Iliamna Lake, Alaska

A new methodology for estimating water flow rate was modified and tested at two island beaches in Iliamna Lake, Alaska, that are used as spawning sites by sockeye salmon Oncorhynchus nerka. Hydrated plaster of Paris, which dissolves at a rate closely correlated with water flow and temperature, was fabricated into cylinders (35-cm2 surface area) and deployed for approximately 48-h intervals underwater at the gravel surface and 15 cm below the gravel surface to water depths of 3.5 m before and during the 3 week sockeye salmon spawning period in August. Water flow was significantly greater in shallower water, both at the gravel surface and within the gravel. Water flow also varied across these spawning beaches at 1.5-m isobaths, and flow was strongly correlated with wind conditions during the deployment period. Use of plaster of Paris cylinders was instructive in identifying relative water flow rates within and among sites that would have been difficult to measure with conventional methodologies but that may be critical to spawning site selection by sockeye salmon and incubation success at these island beaches.

Pattern of Shoreline Spawning by Sockeye Salmon in a Glacially Turbid Lake: Evidence for Subpopulation Differentiation

Alaskan sockeye salmon Oncorhynchus nerka typically spawn in lake tributaries during summer (early run) and along clear-water lake shorelines and outlet rivers during fall (late run). Production at the glacially turbid Tustumena Lake and its outlet, the Kasilof River (south-central Alaska), was thought to be limited to a single run of sockeye salmon that spawned in the lakeˈs clear-water tributaries. However. up to 40% of the returning sockeye salmon enumerated by sonar as they entered the lake could not be accounted for during lake tributary surveys, which suggested either substantial counting errors or that a large number of fish spawned in the lake itself. Lake shoreline spawning had not been documented in a glacially turbid system. We determined the distribution and pattern of sockeye salmon spawning in the Tustumena Lake system from 1989 to 1991 based on fish collected and radiotagged in the Kasilof River. Spawning areas and times were determined for 324 of 413 sockeye salmon tracked upstream into the lake after release. Of these, 224 fish spawned in tributaries by mid-August and 100 spawned along shoreline areas of the lake during late August. In an additional effort, a distinct late run was discovered that spawned in the Kasilof River at the end of September. Between tributary and shoreline spawners, run and spawning time distributions were significantly different. The number of shoreline spawners was relatively stable and independent of annual escapement levels during the study, which suggests that the shoreline spawning component is distinct and not surplus production from an undifferentiated run. Since Tustumena Lake has been fully deglaciated for only about 2,000 years and is still significantly influenced by glacier meltwater, this diversification of spawning populations is probably a relatively recent and ongoing event.

Riverine Spawning by Sockeye Salmon in the Taku River, Alaska and British Columbia

Radio telemetry was used to determine the distribution of sockeye salmon Oncorhynchus nerka returning to spawn in the glacial Taku River in 1984 and 1986, and to locate and characterize spawning areas used by this species. During the study, 253 sockeye salmon were tracked as they moved upriver; 204 of these were followed to spawning areas. Only 37% of the 204 fish traveled to areas associated with lakes; the remaining 63% returned to “riverine” areas – river areas without lakes (42% to the Taku River main stem, 17% to the Nakina River, and 4% to other rivers). Sockeye salmon spawning in riverine areas used a variety of habitat types, including main-river channels, side channels, tributary streams, and upland sloughs. Most (55%) of the radio-tagged fish that returned to the Taku River main stem were tracked to side-channel spawning areas. Half of the 471 adult sockeye salmon sampled in main-stem spawning areas had migrated to sea as juveniles before their first winter. This study showed that many sockeye salmon returning to the Taku River do not depend on lakes, and that riverine sockeye salmon make up a major portion of the run in some river systems.