Behaviour of walleye, Sander vitreus, and largemouth bass, Micropterus salmoides, exposed to different wave intensities and boat operating conditions during livewell confinement

Abstract  During live-release angling tournaments in North America, fish are typically retained in livewells onboard boats during the angling day. Mortality of fish occurs at some tournaments, and wave intensities and livewell conditions may influence mortality levels. This study used two species of fish targeted in live-release angling tournaments in North America (largemouth bass Micropterus salmoides L. and walleye Sander vitreus L.) to quantify the response(s) of fish in livewells to different wave treatments. Video analyses revealed that largemouth bass were active during low intensity disturbances, but during violent boat movements tended to settle to the bottom of the livewell and orient to face the direction of the disturbance. Walleye were less active than bass for all treatments, and additionally did not orient to face the direction of disturbance, consequently contacting the side of the livewell during boat rocking. These results are considered in the context of mortality at live-release tournaments.     

Mortality of larval walleye pollock Theragra chalcogramma in the western Gulf of Alaska, 1988–91

Mortality rates of larval walleye pollock Theragra chakogramma were estimated from larval survey data from 1988 to 1991. Mortality estimates were based on cohort-specific losses between occupations of survey grids. Interannually, estimates of early feeding stage larval mortality rates ranged over an order of magnitude, from 0.045–0.43 day^-1, and declined sharply with age. There is some evidence that mortality rates of early feeding larvae tend to be negatively correlated with temperature and postively correlated with wind mixing.     

Weight-based population analysis: an estimation method

Generally, the individual weight of large fish is measured at landing, and thus, their weight composition is easily obtained. In this paper, we develop a method of population analysis using weight composition of fish, called weight-based population analysis, or WPA. WPA needs data of catch-at-weight, weight composition, weight from the growth and natural mortality M. We apply the method to the walleye pollack fishery in Funka Bay to evaluate its validity. The results show that the population size estimated from WPA reflects the features of population dynamics, and the estimated parameters reflect this walleye pollack fishery. Further, we compare the results of WPA with those of virtual population analysis (VPA) using catch-at-age data. The trend of population dynamics estimated using WPA was comparable to the results of VPA, suggesting that WPA can estimate population size as well as VPA.     

北太平洋狭鳕资源声学评估调查研究

１９９３年夏季使用“北斗”号渔业调查船对北太平洋的狭鳕进行资源声学评估和渔场环境调查，调查覆盖面积达７６万平方公里。调查结果表明：夏季白令海阿留申海盆区５０－２００ｍ为冷水团占据，水温为１．３－４．９℃。狭鳕成鱼主要分布于海盆区东南部及公海区，主要栖息在１７５－２２５ｍ层。在公海区东北部与陆架之间，首次发现大量狭鳕当生幼鱼，主要栖息在８０－１００ｍ层。成鱼和幼鱼均有明显昼夜垂直移动。海盆区狭鳕平均     

Effect of ocean conditions on the cross-shelf distribution of walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus)

Acoustic trawl surveys were conducted in 2000 and 2001 in two troughs located off the eastern coast of Kodiak Island in the Gulf of Alaska as part of a multiyear, multidisciplinary experiment to examine the influence of environmental conditions on the spatial distribution of adult and juvenile walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus). Continuous underway sea surface temperature samples and water column profiles collected in 2000 and 2001 showed the presence of a sharp shelf‐break front in Chiniak Trough and a mid‐trough front in Barnabas Trough. At distances <22 km from shore, the water column was well mixed, whereas a well‐defined mixed layer was present beyond approximately 22 km from shore. Satellite drifter tracks in Barnabas Trough entered along the upstream edge of the trough and appeared to follow the frontal boundary across the middle portion of the trough. A storm in 2001 weakened stratification and cooled surface water temperature by 1.6–2.1°C. Wind mixing associated with the storm event mixed subsurface chlorophyll a to the surface and enhanced nutrients in the surface waters. The storm event revealed spatial partitioning of summer production in Barnabas Trough, with production concentrated in regions inside the mid‐trough front. In contrast, post‐storm summer production was distributed throughout Chiniak Trough. The spatial distribution of walleye pollock and capelin differed and appeared to be related to differences in habitat characteristics. Acoustic survey data identified four acoustic sign types: age‐1 pollock, adult pollock, capelin, capelin–age‐0 pollock mix. The spatial distribution of these four sign types appears to be influenced by the oceanographic and topographic features of the two troughs. Adult pollock were broadly distributed throughout Chiniak Trough, whereas adult pollock were aggregated on the coastal side of the frontal system in Barnabas Trough. In 2000, capelin occurred with age‐0 pollock. In Chiniak Trough, capelin were most abundant along steep topographic gradients at the edges of the trough and in a deep region near Cape Chiniak, whereas the capelin–age‐0 mix (2000) or capelin (2001) concentrations were observed in slope water intrusions over the outer shelf in Barnabas Trough. Results suggest that habitat selection of walleye pollock and capelin are controlled by different processes. Capelin distributions appear to be limited by oceanographic conditions while other factors appear to be more important for pollock.     

Juvenile growth and survival of heat-shocked triploid hybrid saugeyes, Stizostedion vitreum × S. canadense

Triploid hybrid saugeyes may be a desirable stocking alternative because diploid saugeyes are fertile and can compromise the genetic integrity of parental stocks. Four groups of saugeye eggs were heat-shocked 5 min after fertilization for 15 min at 31 °C, which resulted in an 86.7 ± 9.4% triploidy rate and 57.8 ± 24.3% survival to the eyed-stage. Heat shocked and control saugeyes were stocked in two ponds for 40 days at 395 000 ind. ha^–1. Mean length and weight of triploid and heat-shocked diploid saugeyes were greater ( P  < 0.01) than unshocked diploid saugeyes. Survival of heat-shocked saugeyes (22.4%) was lower than unshocked saugeyes (94.7%). Heat-shocked fish ( n =25 851) harvested from ponds were stocked for 153 days in O'Shaughnessy Reservoir, Ohio, USA. Triploids were 34 mm shorter and weighed 41 g less than heat-shocked diploids ( P  < 0.01). Mean length and weight of triploids were comparable with unshocked diploids elsewhere in Ohio.     

Ichthyophonus‐infected walleye pollock Theragra chalcogramma (Pallas) in the eastern Bering Sea: a potential reservoir of infections in the North Pacific

In 2003, the Alaska walleye pollock industry reported product quality issues attributed to an unspecified parasite in fish muscle. Using molecular and histological methods, we identified the parasite in Bering Sea pollock as Ichthyophonus. Infected pollock were identified throughout the study area, and prevalence was greater in adults than in juveniles. This study not only provides the first documented report of Ichthyophonus in any fish species captured in the Bering Sea, but also reveals that the parasite has been present in this region for nearly 20 years and is not a recent introduction. Sequence analysis of 18S rDNA from Ichthyophonus in pollock revealed that consensus sequences were identical to published parasite sequences from Pacific herring and Yukon River Chinook salmon. Results from this study suggest potential for Ichthyophonus exposures from infected pollock via two trophic pathways; feeding on whole fish as prey and scavenging on industry‐discharged offal. Considering the notable Ichthyophonus levels in pollock, the low host specificity of the parasite and the role of this host as a central prey item in the Bering Sea, pollock likely serve as a key Ichthyophonus reservoir for other susceptible hosts in the North Pacific.     

Chum salmon (Oncorhynchus keta) growth and temperature indices as indicators of the year–class strength of age‐1 walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea

Ecosystem‐based fisheries management requires the development of physical and biological time series that index ocean productivity for stock assessment and recruitment forecasts for commercially important species. As recruitment in marine fish is related to ocean condition, we developed proxies for ocean conditions based on sea surface temperature (SST) and biometric measurements of chum salmon (Oncorhynchus keta) captured in the walleye pollock (Gadus chalcogrammus) fishery in the eastern Bering Sea in three periods (July 16–30, September 1–15 and September 16–30). The main purpose of this paper was to evaluate Pacific salmon (Oncorhynchus spp.) growth as a possible indicator of ocean conditions that, in turn, may affect age‐1 walleye pollock recruitment. Marine growth rates of Pacific salmon are the result of a complex interplay of physical, biological and population‐based factors that fish experience as they range through oceanic habitats. These growth rates can, therefore, be viewed as indicators of recent ocean productivity. Thus, our hypothesis was that estimated intra‐annual growth in body weight of immature and maturing age‐4 male and female chum salmon may be used as a biological indicator of variations in rearing conditions also experienced by age‐0 walleye pollock; consequently, they may be used to predict the recruitment to age‐1 in walleye pollock. Summer SSTs and chum salmon growth at the end of July and September explained the largest amount of variability in walleye pollock recruitment indicating that physical and biological indices of ocean productivity can index fish recruitment.     

Year-to-year variations in Bering Sea ice cover and some consequences for fish distributions

The southernmost extension of winter ice cover varies interannually and on longer time scales, reflecting large-scale changes in driving forces, especially in the position and intensity of the winter Aleutian Low Pressure System. A conspicuous pattern is alternating warm and cool periods of several years' duration. These variations in sea ice cover are reflected in the character of a subsurface cold pool, formed as stratification isolates the deeper cold waters from surface exchanges. The cold pool is better developed and more extensive in summers that follow deep southward penetration of winter sea ice. Interannual and decadal-scale variations in the distributions of some fish stocks reflect those of ice and thermal conditions. In particular, the distribution of walleye pollock, Theragra chalcogramma, varies significantly with multiannual cool and warm years while Arctic cod, Boreogadus saida, is only present within the cold pool. The relation among climate variations, sea ice cover, subsurface thermal conditions, and fish distribution provides information on how climate affects marine ecosystems and may also have practical application in predicting fish distributions.