Temperature influences on growth of unfished juvenile Northern cod (Gadus morhua) during stock collapse

Abstract

This study examined growth of unfished juvenile Northern cod (Gadus morhua) off Newfoundland concomitant with stock collapse in the cold early 1990s. Two unpublished data sets were examined, one from collapse‐period trapping sites along the northeast coast of Newfoundland and one from a post‐collapse inshore trawl survey. Cumulative surface and bottom temperatures were significant predictors of growth rates of the young fish with year‐classes born during collapse experiencing slower growth than those born during subsequent warming. Relationships between accrued temperature and growth were consistent across periods, with slow growth of collapse‐period fish reflecting slower accumulation of temperature‐at‐age. Temperature influences were spatially broad‐based with no significant differences in growth rates for fish captured along the entire northeast coast of Newfoundland. Predicted differences in growth rates for collapse versus recovery year‐classes were proportional to cumulative surface temperatures but not cumulative bottom temperatures. Although significant, temperature effects on growth were relatively unimportant at youngest ages. Overall, growth differences between periods were small but large differences occurred between slowest and fastest growing year‐classes. The results suggest initial responses to increasing temperatures were delayed following collapse. We conclude that although temperature was an important determinant of dampened productivity that it alone cannot account for the collapse and slow recovery of the stock. This is the first known study to directly quantitatively link temperature impacts to an unfished component of the Northern cod stock complex during collapse, removing need for implicit assumptions about whether or not cold conditions contributed to the collapse of this iconic fish stock.

Environmental Control on the Productivity of a Heavily Fished Ecosystem

“…Several studies have investigated the role of environmental changes in the collapse of fish stocks on the NL shelves [12,[22][23][24][25][26]. Less work has been directed towards the time period prior to the collapse, when a recently mechanized fleet achieved record-high catches during the warmest, and potentially most productive, period of the last century in the NW Atlantic [27,28].…”

Taxonomic and functional-trait metrics track recovery of demersal fish and shrimp communities following system collapse

“…Other NL cod stocks on the southern Grand Banks and south of Newfoundland also exhibited a >90% decline in spawning biomass between 1967 and 1993 (Myers et al, 1997). While fishing was thought to play a direct role in the collapse (Myers et al, 1996(Myers et al, , 1997, changes in relative abundances of different trophic levels in the food web and colder than average environmental conditions during the 1980s and early 1990s have also been suggested as important drivers of both the collapse and the failure of stocks to rapidly recover following reductions in fishing pressure (Parsons and Lear, 2001;Rothschild, 2007;Halliday and Pinhorn, 2009;Lilly et al, 2013;Buren et al, 2014a;Buren et al, 2014b;Buren et al, 2019;Mullowney et al, 2019). A combination of these two drivers (fishing and environmental changes) has been proposed as a key explanation for the extensive and widespread changes that occurred (e.g., Rose, 2004;Koen-Alonso et al, 2010;Dempsey et al, 2018).…”

A climate index for the Newfoundland and Labrador shelf

“…Created in 1998 following a general consensus that "changes in climate cannot be ignored as an explanation for fluctuations in marine resources" (from the Introduction of the founding document), the AZMP had a mandate to develop and implement an efficient and effective environmental monitoring program for the northwest Atlantic zone and then to maintain it through the years. The creation of the AZMP came as a direct consequence of the collapse of the Atlantic zone groundfish fisheries in the early 1990s, a period characterized by anomalously cold and potentially unfavourable environmental conditions for the stock, although the latter was highly debated (Lear and Parsons, 1993;de Young and Rose, 1993;Hutchings and Myers, 1994;Mullowney et al, 2019).…”