Poor feeding opportunities and reduced condition factor for salmon post-smolts in the Northeast Atlantic Ocean

Abstract

During the last few decades, many wild Atlantic salmon populations have declined dramatically. One possible contributing factor for the decline is reduced prey availability at sea. Here, we examine post-smolt diet and investigate if post-smolts show signs of selective feeding based on 2546 post-smolts sampled from west of Ireland to the northern Norwegian Sea over a 25-year period. We also test for changes over time in stomach fullness, diet, condition factor and body length. There was a clear reduction in condition factor for post-smolts sampled in the Norwegian Sea in the period 2003–2012. The post-smolt stomach fullness was also reduced in the same period. The reduction in condition factor is partly explained by reduced stomach fullness, including a reduction of highly energetic fish larvae and Amphipoda in the diet. Feeding on other prey, such as meso-zooplankton and insects, cannot substitute the high-quality fish larvae and Amphipoda in the post-smolt diet. This is the first study to document how salmon post-smolts feeding in the Norwegian Sea are affected by reduced feeding conditions. Possible causes for the observed changes in post-smolt feeding are ocean warming, decreased primary productivity, and reduced recruitment of important fish larvae.

Ecological regime shift in the Northeast Atlantic Ocean revealed from the unprecedented reduction in marine growth of Atlantic salmon

“…In contrast, the recruitment of Norwegian spring-spawning herring has fluctuated markedly among years, but no high recruitment indices have been reported since 2004. A recent study in the Norwegian Sea supports the hypothesis that salmon post-smolt rely on fish larvae to maintain a high growth rate, showing that condition of Atlantic salmon post-smolt decreased with lower prevalence of fish larvae in their stomachs ( 32 ). A clear drop in condition factor was also observed in the 25-year time series occurring somewhere between 2003 and 2008 ( 32 ).…”

“…A recent study in the Norwegian Sea supports the hypothesis that salmon post-smolt rely on fish larvae to maintain a high growth rate, showing that condition of Atlantic salmon post-smolt decreased with lower prevalence of fish larvae in their stomachs ( 32 ). A clear drop in condition factor was also observed in the 25-year time series occurring somewhere between 2003 and 2008 ( 32 ). The reasons for lack of recruitment of larval fish along the Norwegian coast are unknown, but top-down processes, such as predation ( 33 ), or bottom-up processes due to lack of suitable zooplankton during early life stages ( 34 ) have been proposed.…”

Impacts of a Changing Ecosystem on the Feeding and Feeding Conditions for Atlantic Salmon During the First Months at Sea

“…For post-smolts sampled in the Norwegian Sea, there was a drop in condition factor in the period 2004-2012 and the condition factor remained low thereafter. The drop in condition factor was associated with reduced stomach fullness, indicating poorer prey availability and feeding opportunities during this period (Utne et al, 2021a). A reduced growth of Atlantic salmon from several populations based on scale analyses has also been shown during this period (Jensen et al, 2012;Trehin et al, 2021;Vollset et al, 2022).…”

“…Two large floats attached on each side and a float in the center kept the trawl at the surface during towing. More information about surveys objectives, trawling and biological sampling is given by Utne et al (2021a). Salmon larger than 35 cm before August and larger than 40 cm in August (Gilbey et al, 2021) were excluded from further analyses to ensure that the dataset only included post-smolts.…”

“…The marine survival has in general decreased since the 1960s (Parrish et al, 1998) but information about post-smolt size and feeding conditions in the Northeast-Atlantic prior to 1995 is limited. The range of prey species consumed by post-smolts during their first summer in the ocean, and which prey species were the most important food items, have been identified (e.g., Rikardsen et al, 2004;Haugland et al, 2006;Utne et al, 2021a). However, these data have not been used to address how the post-smolt diet may have changed in time and space, and potentially contributed to the reduced condition and growth of Atlantic salmon at sea.…”