Biogeography of pelagic food webs in the North Pacific

The tufted puffin (Fratercula cirrhata) is a generalist seabird that breeds throughout the North Pacific and eats more than 75 different prey species. Using puffins as samplers, we characterized the geographic variability in pelagic food webs across the subarctic North Pacific from the composition of ~10,000 tufted puffin meals (~56,000 prey items) collected at 35 colonies in the Gulf of Alaska (GoA) and Aleutian Archipelago. Cluster analysis of diet species composition suggested three distinct forage fish communities: (i) in the northern GoA, multiple age‐classes of coastal and shelf residents such as capelin, sand lance and herring dominated the food web, (ii) in the western GoA to eastern Aleutians, the shelf community was dominated by transient age‐0 walleye pollock, and (iii) in the western Aleutians, shelf‐edge and mesopelagic forage species such as squid, lanternfish, and Atka mackerel were prevalent. Geographic patterns of abundance of capelin and sand lance in tufted puffin diets were corroborated by independent research fisheries and diets of piscivorous fish, indicating that puffin diets reflect the local abundance of forage species, not just selection of favored species. Generalized additive models showed that habitat characteristics predict, in a non‐linear fashion, forage species distribution and abundance across two large marine ecosystems. We conclude that major biogeographic patterns in forage fish distribution follow gradients in key habitat features, and puffin diets reflect those patterns.     

Productivity and recovery of forage fish under climate change and fishing: North Sea sandeel as a case study

Forage fish occupy a central position in marine food‐webs worldwide by mediating the transfer of energy and organic matter from lower to higher trophic levels. The lesser sandeel (Ammodytes marinus) is one of the ecologically and economically most important forage fish species in the North‐east Atlantic, acting as a key prey for predatory fish and sea birds, as well as supporting a large commercial fishery. In this case study, we investigate the underlying factors affecting recruitment and how these in turn affect productivity of the North Sea sandeel using long‐term data and modelling. Our results demonstrate how sandeel productivity in the central North Sea (Dogger Bank) depends on a combination of external and internal regulatory factors, including fishing and climate effects, as well as density dependence and food availability of the preferred zooplankton prey (Calanus finmarchicus and Temora longicornis). Furthermore, our model scenarios suggest that while fishing largely contributed to the abrupt stock decline during the late 1990s and the following period of low biomass, a complete recovery of the stock to the highly productive levels of the early 1980s would only be possible through changes in the surrounding ecosystem, involving lower temperatures and improved feeding conditions. To that end, we stress the need for ecosystem‐based management accounting for multiple internal and external factors occurring within the broader context of the ecosystem in which forage fish species, such as sandeel, play an important and integral part.     

Pacific herring (Clupea pallasii) consumption by marine birds during winter in Prince William Sound,Alaska

Following the 1989 MV Exxon Valdez oil spill (EVOS) and subsequent herring population collapse in Alaska's Prince William Sound (PWS), the Pacific herring (Clupea pallasii) fishery was closed. In the 25 yr since EVOS, herring and several herring‐dependent marine bird species have failed to reach pre‐spill population levels. One hypothesis is that intense predation pressure may be inhibiting herring recovery. To inform herring modeling efforts, this study estimated marine bird consumption of juvenile and adult herring in PWS for 10 winters over an 18‐yr period (1989–90 through 2006–2007). Total estimated herring consumption by wintering marine birds averaged 2409 ± 950 t, indicating that avian consumption represents a substantial and inter‐annually variable source of herring mortality. Common murre (Uria aalge) consumed the greatest portion (16–80%) of herring in all years among marine bird species. Juvenile herring biomass consumed annually by common murre was greater than murre consumption of adult herring biomass. Time lag analyses showed that marine bird consumption of adult herring is negatively correlated with the amount of herring spawn observed in subsequent years, but such effects were not observed more than 2 yr. Our models indicate that during years in which herring recruitment is low or bird populations are particularly large, marine birds can consume up to 10% of the annual adult herring biomass. Our results highlight the importance of herring to wintering PWS birds. We propose that future management of herring stocks seeks to reduce negative impacts on marine birds that prey on herring.     

Ecosystem controls of juvenile pink salmon (Onchorynchus gorbuscha) and Pacific herring (Clupea pallasi) populations in Prince William Sound,Alaska

Five years of field, laboratory, and numerical modelling studies demonstrated ecosystem‐level mechanisms influencing the mortality of juvenile pink salmon and Pacific herring. Both species are prey for other fishes, seabirds, and marine mammals in Prince William Sound. We identified critical time‐space linkages between the juvenile stages of pink salmon and herring rearing in shallow‐water nursery areas and seasonally varying ocean state, the availability of appropriate zooplankton forage, and the kinds and numbers of predators. These relationships defined unique habitat dependencies for juveniles whose survivals were strongly linked to growth rates, energy reserves, and seasonal trophic sheltering from predators. We found that juvenile herring were subject to substantial starvation losses during a winter period of plankton diminishment, and that predation on juvenile pink salmon was closely linked to the availability of alternative prey for fish and bird predators. Our collaborative study further revealed that juvenile pink salmon and age‐0 herring exploit very different portions of the annual production cycle. Juvenile pink salmon targeted the cool‐water, early spring plankton bloom dominated by diatoms and large calanoid copepods, whereas young‐of‐the‐year juvenile herring were dependent on warmer conditions occurring later in the postbloom summer and fall when zooplankton was composed of smaller calanoids and a diversity of other taxa. The synopsis of our studies presented in this volume speaks to contemporary issues facing investigators of fish ecosystems, including juvenile fishes, and offers new insight into problems of bottom‐up and top‐down control. In aggregate, our results point to the importance of seeking mechanistic rather than correlative understandings of complex natural systems.     

Migration motivation of cultured Atlantic salmon,Salmo salar L., smolts in relation to size,time of release and acclimatization period

Within the framework of a national sea-ranching programme in Norway a field study of marine release of smolts was conducted in Selsto Bay, west Norway. In March/early April 42 000 cultured Atlantic salmon, Salmo salar L., 10–20cm in body length were divided into two size groups consisting of fish smaller and larger than approximately 14–15 cm and reared in eight tanks, four of each size category. The eight groups were transferred to net pens located in a small marine bay where artificial estuarine salinity gradients had been established. Prior to release from net pens between 13 May and 3 June the fish were exposed to an acclimatization period of either 3 or 7 days. The migratory behaviour of the released smolts was studied by underwater video recordings and divers observations. Fast-migrating schools followed the seabed at 8–10m depth, leaving the bay. Non-migrating fish formed large groups close to the seabed (4–7 m depth), or smaller groups close to surface, in the inner part of the bay. Migratory motivation was higher for the large-size groups and increased during the release period for both size categories. Increasing the acclimatization period from 3 to 7 days may have stimulated the migration of small fish early in the release period but the effect of time of release seemed to be more important for the rest of the experiment. During the release period only small smolts were found in the stomachs of gill-netted predators in the bay. The data indicated that smolt size and time of release strongly influenced the migratory motivation of cultured smolts and that the release strategy may affect predation in the release area.     

Energy dynamics of subyearling Chinook salmon reveal the importance of piscivory to short‐term growth during early marine residence

Variation in prey quantity and quality can influence growth and survival of marine predators, including anadromous fish that migrate from freshwater systems. The objective of this study was to examine the energy dynamics of subyearling Chinook salmon (Oncorhynchus tshawytscha) following freshwater emigration. To address this objective, a population of Chinook salmon and their marine prey were repeatedly sampled from June to September over 2 years in coastal waters off Oregon and Washington. Subyearlings from the same population were also reared under laboratory conditions. Using a bioenergetics model evaluated in the laboratory, we found that growth rate variability in the field was associated more with differences in northern anchovy (Engraulis mordax) consumption and less with variation in diet energy density or ocean temperature. Highest growth rates (2.43–3.22% body weight/day) occurred in months when anchovy biomass peaked, and the timing of peak anchovy biomass varied by year. Our results support a general pattern among subyearling Chinook salmon occurring from Alaska to California that feeding rates contribute most to growth rate variability during early marine residence, although dominant prey types can differ seasonally, annually, or by ecosystem. In the northern California Current, faster growth appears to be associated with the availability of age‐0 anchovy. Identifying factors that influence the seasonal development of the prey field and regulate prey quantity and quality will improve understanding of salmon growth and survival during early marine residence.     

Habitat use of lake trout (Salvelinus namaycush) following species introduction

Abstract – Although introductions of prey species have the potential to significantly alter habitat use by top predatory fish, this aspect has rarely been directly quantified. Introduction of yellow perch (Perca flavescens), a littoral–pelagic prey species, to a small boreal lake previously dominated by littoral cyprinids provided a unique opportunity to examine how a change in forage base influenced habitat use by the sole top predator, lake trout (Salvelinus namaycush). We monitored lake trout pelagic and spatial distribution using acoustic telemetry before (2001) and after (2008) the introduction of perch to determine whether habitat use reflected a deeper, offshore prey community. After accounting for differences in water temperature and dissolved oxygen concentrations between years and the inclusion of a control lake, our data suggest that lake trout habitat use changed after the introduction of yellow perch. Lake trout, on average, were 1.4 m deeper (P < 0.01), reduced their use of littoral habitat by 55% (P = 0.03) and experienced a 71% decrease in home range size (P < 0.01), consistent with a greater offshore habitat overlap between predator and prey after the introduction of yellow perch. This study illustrates how introduced prey species may have a significant influence on habitat use by top predatory fish, while also showing the importance of using direct measurements to quantify behavioural changes.     

Acoustical-optical assessment of Pacific herring and their predator assemblage in Prince William Sound,Alaska

The Pacific herring Clupea pallasi population in Prince William Sound (PWS), Alaska, is both a valuable commercial resource and an important forage species for marine fish and wildlife. Historically, the herring were managed by a combination of age-structured models and egg deposition estimates. When these methods predicted a large return for spring 1993 that failed to materialize, we began surveying with echointegration–purse seine methods. After a decade of acoustic surveys, we show the new approach yields highly precise biomass estimates, which are consistent with historical measures of the miles of beach spawning. When compared, we show the traditional methods overestimated stock biomass, which resulted in harvest rates approaching 40%. In contrast, the acoustic methods are most likely to underestimate biomass. Since the acoustic estimates can be quickly obtained, we recommend their use to set harvest quotas for the fishery in the spring just prior to harvest. The shift from the traditional preseason to inseason management practices for herring in PWS is consistent with the Precautionary Principle by the fact that protection of the spawning population does not rely on the ability of science to predict how the population is changing. Furthermore, synoptic infrared measurements on our night-time acoustic surveys revealed herring to be the most important winter forage to marine birds and wildlife in PWS, including the endangered Steller sea lion Eumetopias jubatus. Given the importance of forage to marine birds and wildlife in the North Pacific during the extended winter conditions (October–March), the implementation of inseason management for herring using echointegration–purse seine techniques may be the most effective method to restore depressed populations of marine birds and mammals in the North Pacific.     

Biotic and abiotic controls on body size during critical life history stages of a pelagic fish,Pacific herring (Clupea pallasii)

Variation in growth and body size during critical life history stages can have important implications for life history schedules and survivorship. For Pacific herring (Clupea pallasii), there is still debate as to whether juvenile body size is governed by density‐dependent or ‐independent processes and few have evaluated whether the relative importance of either process shifts over the course of early ontogeny. We used a unique data set consisting of seasonal measurements of abundance, body size, and spatial distribution within a semi‐enclosed basin of Puget Sound (Washington State, U.S.A.) to measure the relative importance of temperature and cohort abundance on body size at distinct time periods, and evaluated whether density‐dependent habitat shifts might be responsible for density‐dependent growth. Over the 9 years of sampling (2001–2010) midsummer body size was positively related to temperatures experienced during the egg/yolk sac and larval stages and unrelated to cohort abundance. However, fall body size was negatively correlated with abundance and uncorrelated with both midsummer body size and temperature, indicating a shift from density‐independent to density‐dependent control over the course of the growing season. Thus, density‐dependent effects may supplant density‐independent effects exhibited early in herring life history. Our data on spatial distributions of herring and their zooplankton prey indicate that density‐dependent reductions in growth may be explained by density‐dependent habitat shifts that lead to reduce overlap of herring with zooplankton. Evidence of density‐dependent growth in marine fish populations is often attributed to exploitative competition, but our results suggest that these patterns may partly be mediated by density‐dependent distribution expansions in to prey‐poor habitat.     

Seal predation on salmon and forage fish schools as a function of tidal currents in the San Juan Islands, Washington, USA

Tidal patterns in feeding behaviour are known in several upper trophic level predators. Although harbour seal ( Phoca vitulina richardsi ) movements between resting and foraging areas are often correlated with tidal phase, little is known about tidal influences on seal foraging because it is difficult to make direct observations of predation events. This study sought to determine whether harbour seals exhibit tidal patterns in their at-sea distribution, abundance, and foraging success and to discuss how changes in capture rates or prey types might affect the ability of an individual to meet its daily energy requirement. During 1995–97, seal abundance in the water during flooding tides was significantly greater than median daily abundance. Seals aggregated near a channel constriction. Salmon accounted for 50% and 87% of observed captures of single, large fish in 1996 and 1997, respectively. Predation on schooling fishes involved juvenile sandlance or herring. Large-fish captures were episodic (16.9% and 27.5% of observations with nonzero capture rates) and occurred more often on the incoming tide near constricted water flow. Median per capita capture rates were highest in currents during slower flooding (0.31 fish·seal^–1 h^–1). Surface attacks on forage fish schools were more common than large-fish captures (54.0% and 66.7% of observations with at least one attack on forage fish). Night-time and subsurface feeding were not assessed. Given what is known about seal food requirements, tidal differences in capture rates are predicted to have a significant impact on both the hunting strategy and energy intake of individuals. Results support the idea that interactions among tidal currents, topographic features, and fish play a role in structuring marine predator–prey dynamics.