Retrospective analysis of marine growth and relationships to return rates of Penobscot River Atlantic salmon

Abstract

Beginning in the 1980s, return rates of Atlantic salmon to the Penobscot River, Maine U.S.A. declined and have persisted at low levels. This downturn coincided with similar declines in North American and European Atlantic salmon stocks and with changes in the Northwest Atlantic ecosystem. Previous studies investigated whether early marine growth explained the declines, but results varied, with decreased growth associated with declines in European stocks but not North American stocks. In this study, we evaluate whether growth over the entire marine stage is related to Atlantic salmon marine survival. We constructed a growth time series from scales of returned Penobscot River Atlantic salmon spanning periods of varying marine survival. We used ANOVA and post-hoc tests to quantify seasonal growth increment differences and principal component analysis to characterize variability among the suite of growth increments. We observed reduced growth during the second winter and second marine year starting in the 1990s, with compensatory seasonal growth relationships. These results indicate that diminished growth during late marine stages is associated with low return rates in this population.

A multi‐population approach supports common patterns in marine growth and maturation decision in Atlantic salmon (Salmo salar L.) from southern Europe

“…In the literature, we could not find empirical evidence of an ecosystem change at the scale of the North Atlantic basin. Indeed, the sharp decline in early post‐smolt growth (first summer) reported in southern European populations, feeding in the Norwegian sea, was not detected in salmon populations from northern Norway (Vollset et al, 2022) or North America (Barajas et al, 2021; Hogan & Friedland, 2010; Tillotson et al, 2021), at a time when these post‐smolts are expected to be already up at the feeding grounds in the Arctic waters of the White Sea and Barents Sea, and to the Labrador Sea, respectively. The locations of feeding grounds remain imprecisely defined over space and time, as it proves extremely difficult to directly track and catch smolts at sea (especially for small populations).…”

“…Although most of the gain in length occurs during the first summer at sea (Tréhin et al, 2021), adult body length is a result of growth over successive and contrasting seasons. A mechanism of compensatory growth at the individual level (Barajas et al, 2021), or the improvement in growth conditions in the winter season (this study), may compensate for an initial slow growth and attenuate the overall decline in body length after the marine sojourn.…”

“…This advocates extension of our study to the North Atlantic basin scale. However, examination of the available empirical evidence failed to pinpoint the expected similarities in growth trajectories between European and North American populations (Barajas et al, 2021; Hogan & Friedland, 2010). This result is consistent with Tirronen et al (2022) who suggested a non‐stationarity and recent divergence in the ecological response of European and North American salmon populations.…”

“…Salmon growth may show contrasting patterns and trends between summer and winter periods, which may reveal seasonal changes in food availability, spatial distribution, and physiology and maturation status (Tréhin et al, 2021). In addition, growth after the first winter at sea may provide insightful clues about ongoing environmental changes, as reported for North American populations (Barajas et al, 2021). To investigate this seasonal variability, growth patterns were analysed at a seasonal scale across multiple seasons.…”

“…Growth patterns for later periods of the marine sojourn have been much less investigated, although their influence on salmon survival and other life histories may also be critical. For instance, Barajas et al (2021) recently demonstrated that diminished growth during late marine stages was associated with low return rates in the Penobscot River salmon population (Maine, USA).…”