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Newport Laboratory: Fisheries Behavioral Ecology Program

Causes and Consequences of Winter Mortality in Fishes

Examination of the ecology of fishes during winter has lagged well behind study of fishes in summertime. This discrepancy is due to the logistical impediments of winter field work as well as a lingering perception that winter represents a period of stasis in the life history. However, accumulating evidence demonstrates that the overwintering phase is a critical period in the recruitment dynamics of many marine, estuarine, and freshwater fishes, including some Alaskan species.

Fish populations impacted by winter mortality include a number of important North American and European resource species, yet the sources of this mortality remain unidentified in most populations where it has been documented.

For example, the causes of the 1993 episode that reduced spawning biomass of Prince William Sound Pacific herring (Clupea pallasi) by approximately 80% are still unknown because of limited knowledge of environmental conditions during the event and evidence pointing to several alternative explanations. Among the potential sources, thermal stress and starvation have received the most research attention (Fig. 6 below).

figure 6, click to enlarge

Figure 6,  Schematic representation of factors influencing winter mortality in fishes.  Primary Sources of winter mortality (in boxes) are influenced by environmental Conditions (intrinsic, abiotic and ecological) directly or indirectly (indicated by Processes).  Heavy lines indicate relationships having received the most research attention to date.  Likely Interactions between mortality sources are indicated by dashed lines.

Other sources, including predation and pathogens, have significant impacts but have received insufficient attention to date. Designs of more recent laboratory experiments have reflected recognition of the potential for interactions among these co-occurring stressors. One such experiment examined the interactive effects of winter temperature on salinity tolerance, swimming ability, and vulnerability to pathogens of Alaskan coho salmon (Oncorhynchus kisutch).

Geographic patterns in winter mortality are, in some cases, linked to latitudinal clines in winter severity and variability. However, for many freshwater species in particular, the effects of local community structure (predators and prey) may overwhelm latitudinal patterns. Many aspects of marine (and estuarine) systems differ from freshwater systems in ways important to overwintering fishes, the most important being the lack of isolating barriers in the ocean. While open population boundaries allow fish to adopt migration strategies minimizing exposure to thermal stresses, they may retard rates of evolution to local environments. Geographic patterns in the occurrence and causes of winter mortality are ultimately determined by the interaction of regional and local factors.

Winter mortality impacts population dynamics through episodic depressions in stock size and regulation of annual cohort strength. While the former tends to act in a density-independent manner, the latter can be density-dependent as most sources of mortality tend to select against the smallest members of the cohort/population. Determining the density-dependence of winter mortality will be a critical, but difficult, issue to resolve in specific populations. For example, recruitment of Bering Sea walleye pollock appears to be correlated with winter severity, implying a direct density-independent regulation. However, winter mortality is believed to result from increased inter-cohort cannibalism in cold winters due to increased spatial overlap between age groups, a mechanism likely to act, at least in part, in a density-dependent manner.

While some winterkills result in only temporary declines in abundance, others have had long-term impacts by altering the trophic structure and biotic controls on recruitment in the system. Most stock assessment and management regimes have yet to explicitly incorporate variability in winter mortality. However, potential management responses that are beginning to be applied include postponement of cohort evaluation (to after first winter of life), harvest restrictions following mortality events, and habitat enhancement.

Future research should place more emphasis on the ecological aspects of winter mortality including the influences of food web structure on starvation and predation potential. Beyond illuminating an understudied life history phase, studies of overwintering ecology are integral to advancing the primary initiatives in contemporary fisheries science including ecosystem management, habitat evaluation, and impacts of climate change.

By Thomas Hurst

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