Ecological processes influencing mortality of juvenile pink salmon (Oncorhynchus gorbuscha) in Prince William Sound, Alaska

Abstract

Our collaborative work focused on understanding the system of mechanisms influencing the mortality of juvenile pink salmon (Oncorhynchus gorbuscha) in Prince William Sound, Alaska. Coordinated field studies, data analysis and numerical modelling projects were used to identify and explain the mechanisms and their roles in juvenile mortality. In particular, project studies addressed the identification of major fish and bird predators consuming juvenile salmon and the evaluation of three hypotheses linking these losses to (i) alternative prey for predators (prey‐switching hypothesis); (ii) salmon foraging behaviour (refuge‐dispersion hypothesis); and (iii) salmon size and growth (size‐refuge hypothesis). Two facultative planktivorous fishes, Pacific herring (Clupea pallasi) and walleye pollock (Theragra chalcogramma), probably consumed the most juvenile pink salmon each year, although other gadids were also important. Our prey‐switching hypothesis was supported by data indicating that herring and pollock switched to alternative nekton prey, including juvenile salmon, when the biomass of large copepods declined below about 0.2 g m−3. Model simulations were consistent with these findings, but simulations suggested that a June pteropod bloom also sheltered juvenile salmon from predation. Our refuge‐dispersion hypothesis was supported by data indicating a five‐fold increase in predation losses of juvenile salmon when salmon dispersed from nearshore habitats as the biomass of large copepods declined. Our size‐refuge hypothesis was supported by data indicating that size‐ and growth‐dependent vulnerabilities of salmon to predators were a function of predator and prey sizes and the timing of predation events. Our model simulations offered support for the efficacy of representing ecological processes affecting juvenile fishes as systems of coupled evolution equations representing both spatial distribution and physiological status. Simulations wherein model dimensionality was limited through construction of composite trophic groups reproduced the dominant patterns in salmon survival data. In our study, these composite trophic groups were six key zooplankton taxonomic groups, two categories of adult pelagic fishes, and from six to 12 groups for tagged hatchery‐reared juvenile salmon. Model simulations also suggested the importance of salmon density and predator size as important factors modifying the predation process.

Limacina helicinashell dissolution as an indicator of declining habitat suitability owing to ocean acidification in the California Current Ecosystem

“…Although predictive algorithms of V ar have been developed for the CCE using these variables [17,39], they are not valid for nearsurface waters (less than 15 m) and were therefore not applied to our dataset. In preliminary models of V ar , we evaluated multiple regression models where temperature and oxygen were included as predictor variables, but we observed residual error structure at the station level.…”

Effects of Release Timing on the Recovery of Late-Run Chum Salmon in the Okhotsk Sea Coast of Hokkaido, Japan

“…Therefore, sea level diff erences among these sites is used as an index of the strength of the Soya Warm Current and the timing of SST increases in Abashiri Bay. When juvenile chum salmon from the late run are released late because of cold water temperatures in incubating and rearing water and/or low hatchery capacity, the resulting small juveniles would be expected to show reduced off shore movement as shown in the present study, likely due to size-selective mortality (Parker 1971;Healey 1982;Willette et al 2001;Malick et al 2011). Pearcy et al (1989) suggest that rearing chum salmon fry to a larger size may be a useful method to enhance hatchery runs into estuaries, especially if size-selective predation is intensifi ed by retarded growth resulting from high temperatures or lack of prey.…”

“…High mortality of salmon during their early life may be caused by size-selective predation (Parker 1971;Healey 1982;Willette et al 2001;Malick et al 2011). Mortality in chum salmon juveniles was strongly size-selective over the size range from 44 to 55 mm FL (Healey 1982).…”

Hypotheses concerning the decline and poor recovery of Pacific herring in Prince William Sound, Alaska

“…In Fig. 9, the one-dimensional migration route for hatchery pink salmon fry taken from Willette et al (2001) is superimposed on the distribution of age-0 and age-1 herring in July of 1995 and 1996 taken from Norcross (2006), Moffitt (2006) Italics indicates significant difference at P B 0.05 Data from ADFG, Bruce (2006), Dressel (2006), Moffitt (2006) et al (2001). Age-0 herring are in the larval drift in May and June and, after metamorphosis, enter PWS bays and fjords in June and July Stokesberry et al 2000).…”

“…Young-of-the-year salmon stay within the 20-m isobath (Okey and Pauly 1999). Furthermore, most of the hatchery-released pink salmon inhabit the corridor running from Port Valdez to Esther Island and to Knight Island Passage, roughly one-half of the Sound (Okey and Pauly 1999;Willette et al 2001). Juvenile pink salmon can inhabit PWS through October but begin to migrate out of PWS into the coastal waters of the GOA in July .…”