Nicola Hillgruber

Pelagic fish and zooplankton species assemblages in relation to water mass characteristics in the northern Bering and southeast Chukchi seas

This research explores the distributions and community composition of pelagic species in the sub-Arctic and Arctic waters of the northern Bering and central and southern Chukchi seas during September 2007 by linking pelagic zooplankton and fish assemblages to water masses. Juvenile saffron cod (Eleginus gracilis), polar cod (Boreogadus saida), and shorthorn sculpin (Myoxocephalus scorpius) were most abundant in warm, low salinity Alaska Coastal Water (ACW) of the central Chukchi Sea, characterized by low chlorophyll, low nutrients, and small zooplankton taxa. Adult Pacific herring (Clupea pallasii) were more abundant in the less stratified Bering Strait waters and in the colder, saltier Bering Shelf Water of the northern Bering and southern Chukchi seas, characterized by high chlorophyll, high nutrients, and larger zooplankton taxa. Juvenile pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon were most abundant in the less stratified ACW in the central Chukchi Sea and Bering Strait. Abundances of large zooplankton were dominated by copepods (Eucalanus bungii, Calanus glacialis/marshallae, Metridia pacifica) followed by euphausiids (juvenile Thysanoessa raschii and unidentified taxa), whereas small zooplankton were dominated by bivalve larvae and copepods (Centropages abdominalis, Oithona similis, Pseudocalanus sp.). Pelagic community composition was related to environmental factors, with highest correlations between bottom salinity and large zooplankton taxa, and latitude and fish species. These data were collected in a year with strong northward retreat of summer sea ice and therefore provide a baseline for assessing the effects of future climate warming on pelagic ecosystems in sub-Arctic and Arctic regions.

Lack of trophic competition among wild and hatchery juvenile chum salmon during early marine residence in Taku Inlet, Southeast Alaska

Early marine trophic interactions of wild and hatchery chum salmon (Oncorhynchus keta) were examined as a potential cause for the decline in harvests of adult wild chum salmon in Taku Inlet, Southeast Alaska. In 2004 and 2005, outmigrating juvenile chum salmon were sampled in nearshore habitats of the inlet (spring) and in epipelagic habitat at Icy Strait (summer) as they approached the Gulf of Alaska. Fish were frozen for energy density determination or preserved for diet analyses, and hatchery stocks were identified from the presence of thermal marks on otoliths. We compared feeding intensity, diets, energy density, and size relationships of wild and hatchery stocks (n = 3123) across locations and weeks. Only hatchery fish feeding intensity was negatively correlated with fish abundance. In both years, hatchery chum salmon were initially larger and had greater energy density than wild fish, but lost condition in early weeks after release as they adapted to feeding on wild prey assemblages. Diets differed between the stocks at all inlet locations, but did not differ for hatchery salmon between littoral and neritic habitats in the outer inlet, where the stocks overlapped most. Both diets and energy density converged by late June. Therefore, if density-dependent interactions affect wild chum salmon, these effects must be very rapid because survivors in Icy Strait showed few differences. Our study also demonstrates that hatchery release strategies used near Taku Inlet successfully promote early spatial segregation and prey partitioning, which reduce the probability of competition between wild and hatchery chum salmon stocks.

OFFSHORE MARINE OBSERVATION OF WILLOW PTARMIGAN, INCLUDING WATER LANDINGS, KUSKOKWIM BAY, ALASKA

Abstract We report an observation of Willow Ptarmigan (Lagopus lagopus) encountered 8 to 17 km from the nearest shoreline on Kuskokwim Bay, Alaska, on 30 August 2003. The ptarmigan were observed flying, landing on our research vessel, and landing and taking off from the water surface. We also report on one other observation of ptarmigan sitting on the water surface and other marine observations of ptarmigan from the North Pacific Pelagic Seabird Database. These observations provide evidence that Willow Ptarmigan are capable of dispersing across large bodies of water and landing and taking off from the water surface.

Temporal and Ontogenetic Variability in Trophic Role of Four Groundfish Species—Walleye Pollock, Pacific Cod, Arrowtooth Flounder, and Pacific Halibut—around Kodiak Island in the Gulf of Alaska

Abstract Changes in trophic position estimates of commercial fishery catches are used as an ecosystem-based indicator for sustainability, but often these estimates do not incorporate species-specific seasonal feeding dynamics and ontogenetic diet changes. Using stable isotope analysis, we obtained a fine-scale resolution of ontogenetic and temporal (interannual and seasonal) variations in the trophic roles of four commercially and ecologically important groundfish species in the Gulf of Alaska: walleye pollock Theragra chalcogramma, Pacific cod Gadus macrocephalus, arrowtooth flounder Atheresthes stomias, and Pacific halibut Hippoglossus stenolepis. For each groundfish taxon, the nitrogen stable isotope signature (δ15N) increased with total length. In contrast, the lipid-normalized carbon stable isotope signature (δ13C′) significantly varied with size-class only for walleye pollock. There were species-specific differences in trophic position; adult Pacific cod fed at the highest trophic position, and wall...

Effects of Maternal Growth on Fecundity and Egg Quality of Wild and Captive Atka Mackerel

Abstract Trade-offs in energy allocation between growth and reproduction can result in variations in reproductive potential in fish with differing growth patterns. Spawning biomass is often used as a proxy for reproductive potential on the assumption that fecundity is directly proportional to body weight. We examined variations in the reproductive potential of Atka mackerel Pleurogrammus monopterygius by studying the effect of differential growth and condition patterns on fecundity, atresia, and egg energy. Fecundity and egg energy were determined for fish from two geographic areas, Seguam Pass and Amchitka Island, Alaska, and compared with those of fish held in captivity. These Atka mackerel showed distinct differences in growth and condition, with weight at length and length at age being the highest among captive fish, intermediate among fish from Seguam Pass, and lowest among fish from Amchitka Island. Realized fecundity showed that on average captive fish spawned seven batches, fish from Seguam Pass six batches, and fish from Amchitka Island five batches. For wild fish, potential and realized fecundity at length or age was significantly higher at Seguam Pass than at Amchitka Island, whereas the fecundity-at-weight relationship did not differ by area, suggesting that weight is a better predictor of fecundity than length or age. Atresia and batch fecundity by length or weight did not differ by area, suggesting that the variation in fecundity is better explained by the variation in batch number than by batch size. Oocyte dry weight was higher for captive fish than for wild fish, whereas batch order did not significantly affect oocyte dry weight. Increased potential fecundity, realized fecundity, and oocyte quality in Atka mackerel females were strongly related to body size, indicating that growth differences and maternal feeding success impact the fecundity and oocyte quality of Atka mackerel. Therefore, changes in growth and condition patterns need to be taken into account to accurately estimate the reproductive potential of this species.