Donald E. Rogers

Biocomplexity and fisheries sustainability

A classic example of a sustainable fishery is that targeting sockeye salmon in Bristol Bay, Alaska, where record catches have occurred during the last 20 years. The stock complex is an amalgamation of several hundred discrete spawning populations. Structured within lake systems, individual populations display diverse life history characteristics and local adaptations to the variation in spawning and rearing habitats. This biocomplexity has enabled the aggregate of populations to sustain its productivity despite major changes in climatic conditions affecting the freshwater and marine environments during the last century. Different geographic and life history components that were minor producers during one climatic regime have dominated during others, emphasizing that the biocomplexity of fish stocks is critical for maintaining their resilience to environmental change.

EFFECTS OF CHANGING CLIMATE ON ZOOPLANKTON AND JUVENILE SOCKEYE SALMON GROWTH IN SOUTHWESTERN ALASKA

Detecting and forecasting the effects of changing climate on natural and exploited populations represent a major challenge to ecologists and resource managers. These efforts are complicated by underlying density-dependent processes and the differ- ential responses of predators and their prey to changing climate. We explored the effects of density-dependence and changing climate on growth of juvenile sockeye salmon and the densities of their zooplankton prey in the Wood River system of southwestern Alaska. We fit dynamic time-series models to data collected between 1962 and 2002 describing growth of juvenile sockeye, timing of spring ice breakup, and summer zooplankton densities. The timing of spring breakup has moved about seven days earlier now than it was in the early 1960s. Our analyses suggest that most of this shift has been a response to the warm phase of the Pacific Decadal Oscillation that persisted from the mid-1970s to the late 1990s. This progression toward earlier spring breakup dates was associated with warmer summer water temperatures and increased zooplankton (especially Daphnia) densities, which translated into increased sockeye growth during their first year of life. The number of spawning adults that produced each year class of sockeye had a strong negative effect on juvenile sockeye growth rates, so that the size of the density-dependent effect was, on average, twice as large as the effect of spring breakup date. These results highlight the complexity of eco- logical responses to changing climate and suggest that climate warming may enhance growing conditions for juvenile salmonids in large lakes of Alaska.

Variation in life history characteristics and morphology of sockeye salmon in the Kvichak River system, Bristol Bay, Alaska

Abstract Sockeye salmon Oncorhynchus nerka spawn in many streams and along lake beaches of the Kvichak River system in Alaska, but fry from the distinct spawning areas reside in a common nursery habitat, Iliamna Lake. In addition, Kvichak River subpopulations have similar dates of adult entry into fresh water, similar migration distances, and similar spawning dates. These similarities in rearing environments and migratory timing enabled us to test the hypothesis that differences in spawning and incubation habitat alone can promote differentiation in traits associated with reproductive success. River-spawning sockeye salmon tended to be larger at age and older than those spawning along island beaches. Females from rivers were more fecund but had smaller eggs than the beach-spawning females. Males from beaches were deeper-bodied and (in one comparison) had relatively longer lower jaws than males from rivers. The tendency of river-spawning females to mature later than beach spawners may be related to a highe...

DIRECTIONAL SELECTION BY FISHERIES AND THE TIMING OF SOCKEYE SALMON (ONCORHYNCHUS NERKA) MIGRATIONS

The timing of migration from feeding to breeding areas is a critical link between the growth and survival of adult animals, their reproduction, and the fitness of their progeny. Commercial fisheries often catch a large fraction of the migrants (e.g., salmon), and exploitation rates can vary systematically over the fishing season. We examined daily records of sockeye salmon (Oncorhynchus nerka) in the Egegik and Ugashik management districts in Bristol Bay, Alaska (USA), for evidence of such temporally selective fishing. In recent years, the early migrants have experienced lower fishing rates than later migrants, especially in the Egegik district, and the median migration date of the fish escaping the fisheries has been getting progressively earlier in both districts. Moreover, the overall runs (catch and escapement) in the Egegik district and, to a lesser extent the Ugashik district, have been getting earlier, as predicted in response to the selection on timing. The trends in timing were not correlated with sea surface temperature in the region of the North Pacific Ocean where the salmon tend to concentrate, but the trends in the two districts were correlated with each other, indicating that there may be some common environmental influence in addition to the effect of selection. Despite the selection, both groups of salmon have remained productive. We hypothesize that this resilience may result from representation of all component populations among the early and late migrants, so that the fisheries have not eliminated entire populations, and from density-dependent processes that may have helped maintain the productivity of these salmon populations.

Factors affecting marine growth of Bristol bay sockeye salmon

Abstract The growth of Bristol Bay sockeye salmon in freshwater and in the last few months at sea is dependent on their abundance (a negative correlate) and the prevailing water temperature (a positive correlate). In 1990 and 1991, the returning sockeye salmon to Bristol Bay were unusually small relative to their abundances and temperatures and this raised a concern for limited ocean carrying capacity because other stocks of salmon were also very numerous in those years. However, the inclusion of other stocks of sockeye salmon and the abundant runs of Asian chum salmon in a multiple regression analysis did not provide a better predictor of the body size of Bristol Bay sockeye salmon. Growth increments of Nushagak Bay sockeye salmon in their first and second years at sea (from scale measurements) were correlated with temperatures but not with the abundances since 1975. The growth increment in the third year of sea life was uncorrelated with temperature and abundance but was correlated with final adult length. This suggests that concerns for carrying capacity limitations should be placed on the migratory routes of returning adults when the fish are probably most concentrated and their growth most limited by their food supply.

Predation on Sockeye Salmon Fry by Juvenile Coho Salmon in the Chignik Lakes, Alaska: Implications for Salmon Management

Abstract -The consumption of recently emerged sockeye salmon Oncorhynchus nerka by juvenile coho salmon O. kisutch in Chignik Lake, Alaska, was estimated for 15 May–5 August, 1985–1987. Estimated daily consumption of sockeye salmon fry by individual coho salmon in Chignik Lake, based on a stomach evacuation method, increased from about 2.0 fry in late May to 3.3 fry in June, then it declined in July and early August to 1.1 fry. Average consumption during each year, based on a bioenergetic approach, was within 14% (range, 15–20%) of the stomach evacuation method estimates, whereas bioenergetic estimates on a given day generally were within 30% of the stomach evacuation method estimates. Few coho salmon were captured in nearby Black Lake, where consumption of sockeye salmon fry per juvenile coho salmon was low. Estimates of sockeye salmon fry consumed by coho salmon in Chignik Lake, based on two independent estimates of juvenile coho salmon abundance, were 68 million, 24 million, and 78 million, which repre...

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...

Genetic and Phenotypic Evidence of Reproductive Isolation between Seasonal Runs of Sockeye Salmon in Bear Lake, Alaska

Abstract The effective conservation of salmonids requires the recognition and preservation of populations that are diverse in genetic composition and life history. The management of sockeye salmon Oncorhynchus nerka in Bear Lake, Alaska, is based on the presumption that there are two, genetically isolated seasonal runs that exhibit a bimodal escapement pattern. We investigated the genetic composition and life history of the putative early and late runs in two consecutive years. Significant allele frequency differences at six microsatellite loci demonstrate restricted gene flow between the early and late runs (F ST = 0.017). There were also significant, and presumably adaptive, differences between the runs with respect to body weight, somatic weight, ovary weight, and egg size among females after correction to equal body sizes. Further, scale pattern analysis revealed highly significant differences in the growth-at-age patterns of these runs. These results demonstrate that there are significant genetic dif...

Stock Origins of Chinook Salmon in Incidental Catches by Groundfish Fisheries in the Eastern Bering Sea

Abstract An estimated 386,946 chinook salmon Oncorhynchus tshawytscha were taken in incidental catches by foreign and joint-venture groundfish vessels operating in the Bering Sea portion of the U.S. exclusive economic zone, 1977–1986. Concern about the effect of incidental catches on the salmon fisheries of western Alaska provided the impetus for a study to determine the stock origins of chinook salmon in these catches. The proportions of regional (Asia, western Alaska, central Alaska, and southeast Alaska–British Columbia) and western Alaskan subregional (Yukon, Kuskokwim, and Bristol Bay) stocks of Chinook salmon in 1979, 1981, and 1982 samples collected by U.S. foreign fishery observers were estimated by scale pattern analysis. Estimated proportions ofthe regional stocks averaged 60% western Alaska, 17% central Alaska, 14% Asia, and 9% southeast Alaska-British Columbia. Western Alaska, which included Canadian Yukon fish, was the predominant regional stock in most age-group, time, and area strata. Stock...

Stock Origins of Chinook Salmon in the Area of the Japanese Mothership Salmon Fishery

Abstract The record catch of 704,000 chinook salmon Oncorhynchus tshawytscha by the Japanese mothership salmon fishery in 1980 intensified concern about the effect of high seas interceptions of salmon reared in North America. The goal of this study was to update and refine estimates of the relative proportions of Asian and North American chinook salmon stocks in the mothership fishery area in the Bering Sea and north Pacific Ocean. Linear discriminant analysis of scale pattern data was used to classify samples of immature chinook salmon aged 1.2 (one winter in fresh water, two winters at sea) from the area 46-62°N, 160°E-175°W in June and July 1975-1981 to four regions: Asia, western Alaska, central Alaska, and southeastern Alaska-British Columbia. Western Alaska, which included Canadian Yukon stocks, was further subdivided into three subregions: Yukon River, Kuskokwim district, and Bristol Bay. Overall classification accuracies averaged 74, 79, and 86%, respectively, in four-, three-, and two-category re...