BOOKS RECEIVED

Ten specimens of the codfish Theragra finnmarchica(Teleostei, Gadidae) were known to science until 2003. Collections of 31 additional specimens made off north Norway between February 2003 and April 2004 provide information on the geographical and vertical distribution of this assumed rare species.     

Shifting the balance between foraging and predator avoidance: the importance of food distribution for a schooling pelagic forager

It is widely held that when predator avoidance conflicts with other activities, such as feeding, avoidance of predators often takes precedence. In this study, we examine how predation risk and food distribution interact to influence the schooling behavior and swimming speed of foraging juvenile walleye pollock, Theragra chalcogramma. Fish were acclimated to either spatially and temporally clumped, or spatially and temporally dispersed food for 3 weeks. Fish were then monitored while feeding in the absence and presence of predatory sablefish, Anoplopoma fimbria. Fish foraging for clumped food swam rapidly in a loose school when predators were absent, but swam more slowly and adopted more cohesive schooling in the presence of predators, trading-off foraging opportunity for decreased vulnerability to predators. Fish foraging for dispersed food swam about slowly and did not engage in cohesive schooling in either the absence or presence of predators. These fish accepted greater predation risk in order to continue foraging, suggesting that the cost of schooling, in terms of decreased foraging opportunity, was greater when food was dispersed than when it was clumped. This lower responsiveness to predators among fish receiving dispersed food demonstrates that predator avoidance does not always take precedence over other activities, but rather, that a balance is maintained between predator avoidance and feeding, which shifts as food distribution changes.     

SUMMER FEEDING HABITS OF DALL'S PORPOISE, PHOCOENOIDES DALLI, IN THE SOUTHERN SEA OF OKHOTSK

Stomach contents were analyzed from 85 Dall's porpoises, Phocoenoides dalli , taken in the southern Sea of Okhotsk during the summer of 1988. Thirteen species of fish and five species of cephalopods were identified. Fishes comprising 9 families were predominant and made up 79.9% of the total number of prey, with an overall occurrence in the stomachs of 100%. Three families of cephalopods made up 20.1% of the total number of prey ingested, with an overall occurrence of 74%. The Japanese pilchard, Sardinops sagax melanostictus , was the primary prey and represented 72.0% of the total prey in 97.3% of the stomachs examined. The gonatid squid, Berryteuthis magister , ranked second and made up 16.9% of the total prey in 61.6% of the stomachs. Walleye pollock, Theragra chalcogramma , was the third most abundant prey item consumed and represented 6.0% of the total, with an occurrence of 53.4%. Composition of the sample by the calculated total mass and caloric intake supports the numeric findings on the relative importance of the three major prey species to the DaIl's porpoise diet. The Japanese pilchard was clearly dominant using these indices and constituted 65.4% of the total mass and 80.7% of the caloric intake. Berryteuthis magister made up 26.8% of the mass and 15.5% of the caloric intake. Walleye pollock contributed the least to the diet with 7.9% of the total mass and 3.8% of the caloric intake. Volumetric data indicate that porpoises are feeding heavily on Japanese pilchard throughout the daylight hours. Berryteuthis magister was probably fed upon in the late night or early morning hours. Occurrence of both Dall's porpoise and Japanese pilchard in the coastal waters of the southern Sea of Okhotsk is seasonal. The summer movements of Dall's porpoise into this region may be related to the seasonal migration and abundance of the Japanese pilchard.     

Interannual differences in growth and hatch date distributions of age-0 year walleye pollock Theragra chalcogramma (Pallas) sampled from the Shumagin Islands region of the Gulf of Alaska, 1985 – 2001

Daily increments of age-0 year walleye pollock Theragra chalcogramma otoliths from 1985 to 2001 were examined for interannual variability in growth and hatch dates. Fish were collected in summer and autumn surveys near the Shumagin Islands in the western Gulf of Alaska (GOA). Hatch date distributions of these fish were compared with hatch dates of larvae from spawning aggregations located in Shelikof Strait and the Shumagin Islands based on their spawning times. The hatch date distributions of age-0 year fish captured from the Shumagin Islands area were similar to those observed for larvae from the Shelikof Strait spawning group. Age-0 year fish whose hatch dates corresponded to the Shumagin Islands spawning, which occurred earlier in January and February, were not found. Sea surface temperature was associated with variability in hatch date distribution and growth.     

The taxonomic status of Theragra finnmarchica Koefoed, 1956 (Teleostei: Gadidae): perspectives from morphological and molecular data

The gadoid fishes Theragra finnmarchica from the coast of northern Norway and Theragra chalcogramma from the north‐east Pacific could not be taxonomically discriminated by sequencing the mitochondrial cytochrome c oxidase I (COI) gene. However, the taxa differed in 10 of 42 non‐osteological and in four of 17 osteological characters. Ontogenetic changes were found for 16 of the examined non‐osteological characters. Five taxon‐specific characters, unaffected by ontogenetic scaling, were examined by discriminant analysis. A 100% correct classification was obtained from a discriminant function based on number of pyloric caeca and number of gill rakers on the upper part of the gill arch. However, all examined characters overlapped between the two taxa, except the number of pyloric caeca. On the basis of their genetic similarity and slight morphological differences, T. finnmarchica should be considered a junior synonym of T. chalcogramma.     

Assessing the condition of walleye pollock Theragra chalcogramma (Pallas) larvae using muscle-based flow cytometric cell cycle analysis

Flow cytometric cell cycle analysis was used to determine the fraction of muscle cells in the S and G2 phases of the cell cycle, which were used as covariates with temperature and standard length, in a laboratory-developed model to assess the physiological condition of wild walleye pollock, Theragra chalcogramma, larvae. The assay was calibrated to the range of temperatures larvae are likely to encounter in the eastern Bering Sea, and it was sensitive to changes in condition within 3 days of starvation. The S and G2 phases of the cell cycle gave an indication of larval walleye pollock condition. Healthy larvae had a larger fraction of cells in the S phase than G2 phase, and unhealthy larvae had a larger fraction of cells in the G2 phase than the S phase. Validation tests showed that the model classified 75% to 83% of the larvae correctly. The assessment of the condition of walleye pollock larvae collected from the southeastern Bering Sea in 2007 indicated that unhealthy larvae were located on the continental shelf (6%), and this may be due in part to the coldest temperatures occurring there and less abundant prey. In the continental slope/ocean basin waters, where prey levels were higher and temperatures warmest, no larvae in unhealthy condition were found.     

Scavenging on discarded saury by demersal fishes off Sendai Bay, northern Japan

Pacific saury Cololabis saira were found in 24 out of 1314 stomachs of demersal fishes collected by bottom trawling at 245–393 m depths. The same area was occupied by the stick-held dipnet fishery for saury, in which a landing limitation was enforced to avoid overfishing. The fish species containing saury were: Pacific cod Gadus macrocephalus , walleye pollock Theragra chalcogramma , and oilfish Ruvettus pretiosus . These demersal species would not encounter living saury naturally, which normally are distributed above thermoclines. The ingested saury were smaller than market size which suggests that the saury from fish stomachs were discarded by fishermen because of small sizes to maximize profits under the landing limitation.
The scavenged saury made up 41.5, 24.6 and 77.7% of the diets of large-sized (>30cm)Pacific cod, walleye pollock and oilfish, respectively in terms of DW composition. The extent to which discarded saury contributed to the total diet in the demersal fish assemblage, calculated by considering species composition and diets of bottom fishes, was 21.8% of the total diet. The discarded saury seemed to compensate the less productive feeding environment during autumn for the bottom fishes.     

Gut evacuation of walleye pollock larvae in response to feeding conditions

Gut residence times of first-feeding walleye pollock larvae were measured at 6°C in continuous and discontinuous feeding regimes. Larvae fed tagged copepod prey evacuated their guts more quickly in continuous feeding as compared to the discontinuous feeding treatment. The mean gut residence time was estimated to be 5.0 h for larvae feeding continuously. Gut evacuation by larvae fed tagged prey and then isolated without food (discontinuous treatment) was slower and more variable, with an estimated gut residence time exceeding 8.0 h. Field and laboratory observations suggest that the larval fish gut may be modeled as an intermittent plug-flow reactor (PFR) in response to diel feeding patterns. The cyclical nature of gut dynamics has implications for gut content analyses and the estimation of daily food rations.     

Sex-biased egg cannibalism in spawning walleye pollock: the role of reproductive behavior

We studied inadvertent egg cannibalism in spawning stocks of walleye pollock, Theragra chalcogramma, in the Gulf of Alaska and the Bering Sea between 1986 and 1996. Male pollock had on average 3 times more eggs in their stomachs than females. For both sexes adult fish of average body length had more eggs in their stomach than did smaller or larger fish. When maturity of fish was taken into account, actively spawning males had the highest numbers of eggs in their stomachs. We found weak evidence for a diel variation; the number of eggs per stomach was high in both sexes during the day and lower during the night. We suggest two possible explanations for this phenomenon. Sex-biased egg cannibalism may reflect the differential time spent in layers of high egg densities. Hydroacoustic and trawl catch data from both areas suggest that males aggregate deeper than females. Spawning takes place in the deeper layers of the fish aggregation, so males spend more time in high egg densities. Alternatively, males may be more active than females and increased gill ventilation and/or drinking rates may be responsible for the differences in egg cannibalism.