Towards a balanced presentation and objective interpretation of acoustic and trawl survey data, with specific reference to the eastern Scotian Shelf

Abstract

A debate has developed over the ecosystem consequences following the collapse of Atlantic cod throughout the coastal waters of eastern Canada. The explosive increase in pelagic fish abundance in scientific bottom-trawl catches on the eastern Scotian Shelf has been interpreted as being due to either (i) a "pelagic outburst" of forage fish abundance resulting from predator release or conversely (ii) a change in pelagic fish vertical distribution leading to a "suprabenthic habitat occupation" thereby increasing their availability to bottom trawls. These two interpretations have diametrically opposing ecological consequences and suggest different management strategies for these important forage fish species. We argue that an objective evaluation of the available evidence supports the hypothesis that the abundance of forage fish has not increased in response to the demise of cod and other top predators, and the reliance on a single sampling gear with low catchability has biased and will continue to bias the interpretation of demographic trends of pelagic fish populations. We advocate that multiple sampling technologies providing alternative perspectives are needed for the monitoring and management of the various trophic levels if we are to achieve a balanced and objective understanding of marine ecosystems.Résumé : Un débat a vu le jour concernant les conséquences écosystémiques de l'effondrement des stocks de morue au travers les eaux côtières de l'est du Canada. L'augmentation fulgurante de l'abondance des poissons pélagiques dans les prises scientifiques au chalut de fond dans la partie est de la plate-forme Néo-Écossaise a été interprétée comme étant le fait soit (i) d'une « explosion pélagique » de l'abondance de poissons fourrage résultant de la baisse des prédateurs ou, à l'inverse, (ii) d'un changement de la répartition verticale des poissons pélagiques ayant mené à leur « occupation des habitats suprabenthiques », accroissant leur disponibilité pour les chaluts de fond. Ces deux interprétations ont des conséquences écologiques diamétrale-ment opposées et nécessiteraient des stratégies de gestion différentes pour ces espèces de poissons fourrage importantes. Nous arguons qu'une évaluation objective des données probantes disponibles appuie l'hypothèse selon laquelle l'abondance des poissons fourrage n'a pas augmenté en réponse à l'effondrement de la morue et d'autres prédateurs de niveau trophique supérieur, et que le recours à un seul engin d'échantillonnage caractérisé par une faible capturabilité a faussé et continuera de fausser l'interprétation des tendances démographiques des populations de poissons pélagiques. Nous soutenons que plusieurs techniques d'échantillonnage offrant différentes perspectives sont nécessaires à la surveillance et la gestion des différents niveaux trophiques et pour en arriver à une compréhension équilibrée et objective des écosystèmes marins. [Traduit par la Rédaction]

The role of sand lances (Ammodytes sp.) in the Northwest Atlantic Ecosystem: A synthesis of current knowledge with implications for conservation and management

“…Further, changes in higher‐level predator abundance can create strong top‐down pressures that control NWA Ammodytes dynamics in areas where they are concentrated. Predatory release due to overfishing of Atlantic cod ( Gadus morhua) and other piscivorous fishes was one explanation for observed population increases in NWA Ammodytes in Canadian waters from 1990 to 2010 (Frank et al, 2013; Frank, Petrie, Fisher, & Leggett, 2011); however, this remains an open question as changes in the vertical distribution of pelagic forage fishes provides an alternative explanation, and gear bias may have confounded interpretation of demographic trends (Jech & McQuinn, 2016; McQuinn, 2009).…”

Drivers of Atlantic herring decline and evidence basis for fisheries closures and rebuilding plans

“…For example, across studies in the North Atlantic, herring recruitment is represented variously as an index or normalized with respect to biomass; biomass can refer to age-0, age-2, or other age groups; and temperature is reported for varying depths and clines (or not further defined). This is problematic because (1) environmental variables can have net impacts that are age specific due to the dominance of different mechanisms at different ages (for example, the complex effect of temperature on fallspawning herring described above); and (2) ecosystem nonstationarities have different effects on different proxies for quantities of common interest (for example, the collapse of benthic predator Atlantic cod (Gadus morhua) increased the vertical distribution of pelagic fishes and hence bottom trawl indices while real stocks remained constant or decreased) (Frank et al 2005;McQuinn and Jech 2009;Jech and McQuinn 2016). While most analysis suggests that herring biomass is likely to remain low regardless of recruitment scenarios, investigations into the environmental controls on recruitment and abundance have not converged on a consistent mechanistic explanation.…”

Fish distribution in three dimensions around the Block Island Wind Farm as observed with conventional and volumetric echosounders

“…For example, the NEFSC bottom trawl survey provides data for population assessments of 35 species, with 21 species' assessments being completely or heavily reliant on those data (NEFSC, unpublished data). Due to a variety of factors (such as navigational safety concerns and maneuverability while deploying scientific gear and instruments), most of these NOAA surveys likely will not be able to traverse or sample in OWDAs, so the NEFSC has initiated efforts to evaluate the impact of OWDAs on stock assessments as well as to develop alternative methods, such as optical (e.g., Miller et al 2019) and acoustical technologies (e.g., Jech and Sullivan 2014;Jech and McQuinn 2016), to collect the necessary data for stock assessments (Hare et al 2022).…”