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Fish distribution is strongly influenced by temperature, with populations predicted to respond to ongoing changes in ocean climate by adapting distribution to maintain preferred thermal habitats. Over the last several decades, the Northwest Atlantic off Newfoundland, Canada has experienced wide variation in temperature and significant changes in the structure of the marine community. A period of particularly cold ocean conditions from the mid-1980s to mid-1990s saw Greenland halibut occupying deeper waters, and moving farther offshore and southward. Distributional shifts across periods of varying thermal conditions were most pronounced in the youngest ages. While no specific nursery areas were evident, these young fish typically occurred in shallower waters where temperatures were more variable, moving deeper with age. Sensitivity to changing ocean climate may vary with age. This adaptation to shifting temperatures suggests that this species is likely to alter its distribution in the face of continued changes in ocean climate. Age-specific differences in response to changing temperature have potential implications for ecosystem interactions and fisheries management. [ABSTRACT FROM AUTHOR]

Acoustic methods were tested as an alternative to catch data to measure size spectra of freshwater fish communities in Lac du Bonnet (LdB), Manitoba, Canada. Eleven daytime hydroacoustic surveys conducted in 2011 and 2012 enabled length-frequency spectra of fish communities to be calculated within LdB. Fish echoes tracked and converted to length (>5 cm) formed size spectra with significant linear regressions in all cases ( p values < 0.05, R2 values from 0.87 to 0.99). Fish abundance (spectral height) was greater in 2011 (mean ± SD = 0.19 ± 0.15) than in 2012 (-0.19 ± 0.20), decreasing through the summer sampling season. Both abundance and size composition (spectral slope) of fish communities differed between three basins of varying bathymetry, with steepest slopes and lowest heights associated with the shallowest waters. We conclude that acoustically derived size spectra provide an efficient means to detect and monitor fish community dynamics over varied spatial and temporal scales in aquatic ecosystems where fish are not highly aggregated and hence single acoustic targets predominate. [ABSTRACT FROM AUTHOR]

Mobile hydroacoustic surveys are increasingly used to assess the distribution and abundance of freshwater fish; yet, fish may avoid moving vessels, potentially introducing bias in these assessments. In this study, avoidance in boreal lakes and reservoirs was quantified by developing a simple method based on paired drift:drift (D:D) and drift:motor (D:M) transects. Two systems in eastern Manitoba, Canada were studied: Lac du Bonnet reservoir and Nopiming. Acoustic data were collected using a digital DTX echosounder (BioSonics, Seattle, WA, USA), with a downward facing 200-kHz split-beam transducer, deployed from 5.5-m vessels (Boston Whalers) modified for acoustic research. An avoidance coefficient ( Ac) was developed based on comparisons of acoustic fish densities while the vessel moved over the same transects by drifting, and by motoring at survey speeds. Ac did not differ significantly from 1 (no avoidance) at Nopiming (median of 0.81, n = 13), but did at Lac du Bonnet (median of 0.51, n = 31, P < 0.05). Variability in Ac was as high in transect pairs and was unrelated to fish depth (mean 6.9 m at Lac du Bonnet; 13.1 m at Nopiming) or survey speed (up to 3.70 m·s−1, 7 knots). Results indicated that fishes did not dive in the presence of the motoring vessel, nor was avoidance size-based. We did not detect any evidence of fish attraction to our drifting vessel. Our results suggest that boat avoidance during acoustic surveys of shallow boreal lakes may vary in relatively similar water bodies but can be quantified experimentally. [ABSTRACT FROM AUTHOR]

Starvation is a ubiquitous process in nature as all animals depend on finite resources to survive. Limited food resources can lead to starvation-induced mortality and, depending on the scale of the limitation, population-level consequences may emerge. However, attempts to model starvation-induced mortality are rare as it is difficult to isolate the effects of specific sources of natural mortality. Using commonly collected data on body condition, we develop a statistical approach to index starvation-induced mortality by estimating the proportion of individuals experiencing severe emaciation in a fish population. This approach accounts for seasonal patterns in body condition, allowing data collected at any time of year to be included. Using Canada's Northern cod (Gadus morhua) stock as a case study, we found a positive association between rates of natural mortality estimated by an integrated population model and the index of mortality derived from proportions of cod in poor condition. This index is also associated with the availability of two key prey species, capelin (Mallotus villosus) and Northern shrimp (Pandalus borealis). Taken together, our results indicate that starvation-induced mortality represents an important component of the natural mortality experienced by Northern cod, supporting the idea that insufficient food contributed to the collapse of the stock in the early 1990s and, subsequently, limited its recovery. These implications highlight the practical relevance of explicitly considering prey availability (e.g., through body condition or prey abundance indices) to improve stock assessment modeling and aid the development of actionable ecosystem-based fisheries management strategies. [ABSTRACT FROM AUTHOR]