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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, [R.sup.2] 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. Des methodes acoustiques ont ete mises a l'essai comme solution de rechange aux donnees sur les prises pour mesurer les spectres de tailles de communautes de poissons d'eau douce dans le lac du Bonnet (LdB), au Manitoba (Canada). Onze leves hydroacoustiques diurnes menes en 2011 et 2012 ont permis de calculer les spectres de frequences des longueurs des communautes de poissons dans le LdB. Les echos de poissons suivis et convertis en longueur (>5 cm) formaient des spectres de tailles donnant des regressions lineaires significatives dans tous les cas (p < 0,05; [R.sup.2] = 0,87-0,99). L'abondance des poissons (hauteur des spectres) etait plus grande en 2011 (moyenne ± ecart-type = 0,19 ± 0,15) qu'en 2012 (-0,19 ± 0,20), montrant une diminution au fil de la saison d'echantillonnage estivale. Tant l'abondance que la composition par taille (pente des spectres) des communautes de poissons etaient differentes pour trois bassins de bathymetrie distincte, les pentes les plus grandes et les hauteurs les plus faibles etant associees aux eaux les moins profondes. Nous concluons que les spectres de tailles obtenus de leves acoustiques constituent un moyen efficace pour detecter et surveiller la dynamique des communautes de poissons a differentes echelles spatiales et temporelles dans les ecosystemes aquatiques dans lesquels les poissons ne sont pas tres agreges et oU dominent donc les cibles acoustiques individuelles. [Traduit par la Redaction], Introduction Body size is a fundamental property of all organisms, being linked to life history traits and physiological properties, including production (Sheldon et al. 1972), metabolic rate (Gillooly et al. [...]

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]

The current stock assessment model for American plaice (Hippoglossoides platessoides) on the Grand Bank of Newfoundland (NAFO Divisions 3LNO) is a virtual population analysis (VPA). This model does not account for the considerable uncertainty about the landings data for this stock. Retrospective patterns have also been noted in the current assessment with overestimation of spawning stock biomass (SSB) and underestimation of fishing mortality (F). Via a thorough model selection process, we develop a state-space stock assessment model (SSM) for this stock that accounts for the uncertainties in the landings data and reduces the retrospective patterns. Our SSM fit the data well, with overall trends in SSB and average F (ages 9--14) similar to those estimated from the current VPA. The retrospective patterns for the SSM were reduced for both SSB and average F which should lead to the provision of better scientific advice for the management of this stock. An important result from our analysis suggests that the current assumption for natural mortality (M) in the stock assessment model may be too low. The lack of recovery of the stock of American plaice on the Grand Bank has often been attributed to overfishing, however fixing M within the model to be lower than is reasonable may be over-estimating the relative impact of F and subsequently over-stating the contribution of fishing mortality to the lack of recovery of the stock. [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]

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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]