Selection due to nonretention mortality in gillnet fisheries for salmon

Abstract

Fisheries often exert selective pressures through elevated mortality on a nonrandom component of exploited stocks. Selective removal of individuals will alter the composition of a given population, with potential consequences for its size structure, stability and evolution. Gillnets are known to harvest fish according to size. It is not known, however, whether delayed mortality due to disentanglement from gillnets exerts selective pressures that reinforce or counteract harvest selection. We examined gillnet disentanglement in exploited populations of sockeye salmon (Oncorhynchus nerka) in Bristol Bay, Alaska, to characterize the length distribution of fish that disentangle from gillnets and determine whether nonretention mortality reinforces harvest selection and exerts common pressures according to sex and age. We also evaluated discrete spawning populations to determine whether nonretention affects populations with different morphologies in distinct ways. In aggregate, nonretention mortality in fish that disentangle from gillnets counters harvest selection but with different effects by sex and age. At the level of individual spawning populations, nonretention mortality may exert stabilizing, disruptive, or directional selection depending on the size distribution of a given population. Our analyses suggest nonretention mortality exerts significant selective pressures and should be explicitly included in analyses of fishery-induced selection.

Keywords: conservation biology; contemporary evolution; fisheries management; life history evolution; phenotypic plasticity; population dynamics.

Figure 1

Photographs of gillnet marking in male (left) and female (right) sockeye salmon due to disentanglement in commercial gillnet fisheries [Supporting Information, Appendix S1, Fig. A1 displays gillnet marking as apparent at sexual immaturity (sampling at entry to freshwater system) in contrast to gillnet marking at sexual maturity (sampling at natal streams)].

Figure 2

Map of Wood River system, Bristol Bay, Alaska. The main map displays the Nushagak fishing district and constituent watersheds (Wood River watershed outlined). The enlargement displays the sampling location for the aggregate Wood River system stock (Wood River counting tower), and the sampling locations for the 10 stream spawning populations surveyed in this study (⋆).

Figure 3

Histograms of estimated numbers at length (10 mm length bins) for the total run, harvest, escapement, and disentangled fish in the escapement for the aggregate Wood River system stock (mean, 2006–2008). Density plots on the right contrast the shape and location of the length distributions of harvested, escaped, and disentangled fish. (Supporting Information, Appendix S5, Figs E1–E3 display year-specific length distributions).

Figure 4

Selection pressure due to mortality via harvest (top graphs) and gillnet disentanglement (bottom graphs), contrasting harvested and disentangled fish to preharvest fish as a ratio of proportion at length. The preharvest stock is scaled to a uniform distribution (proportional deciles by length) and represented by the horizontal line at 1.0. Histograms display the relative densities for harvested and disentangled fish (the ratio of the relative proportion harvested/disentangled at length to the preharvest proportion at length), scaled to the uniform preharvest stock. In the case of identical distributions, the histogram would be uniform at 1.0.

Figure 5

Length frequency area plots of the reconstructed aggregate Wood River system stock (total run, by sex and year) with mortality attributed to harvest extraction and nonretention. Nonretention mortality at length was estimated from the length distribution of disentangled fish, annual incidence of gillnet disentanglement in the escapement, and an estimated prespawning mortality rate of 50% for disentangled fish in the escapement. Total run is represented by the combined area. Total escapement is the area minus harvest mortality. The spawning population (effective escapement) is the total run minus harvest and nonretention mortality.

Figure 6

Standardized selection differentials (2006–2008) related to harvest mortality (top panels), nonretention mortality (middle panels) and totaled fisheries-induced selection (lower panels). Error bars represent 95% confidence intervals. Harvest selection produces a decrease in mean length in both sexes and is significant in most years. Nonretention selection produces an increase in length for both sexes but with higher variance in males, largely because nonretention affects males at both extremes of their length range. Total fisheries-induced selection is negative in most years but in years where harvest selection is relatively weak, the selective effects of nonretention mortality are enough to neutralize the selective effects of harvest.

Figure 7

Top panels (boxplots) show individual stream populations as categorized into small- and large-stream types. Lower panels display the length frequency distributions for unmarked and gillnet marked fish that escape to natal streams (histograms) contrasted to length frequency distributions by age (years in ocean residence) of fish in these populations (solid and dashed lines). Arrows suggest how mortality due to gillnet disentanglement may exert directional, stabilizing, or disruptive selection in these population types (Supporting Information, Appendix S6 displays length distributions and selection pressures for individual streams).