Evidence for Size‐Selective Mortality after the First Summer of Ocean Growth by Pink Salmon

Abstract

Pink salmon Onchorhynchus gorbuscha with identifiable thermal otolith marks from Prince William Sound hatchery release groups during 2001 were used to test the hypothesis that faster-growing fish during their first summer in the ocean had higher survival rates than slowergrowing fish. Marked juvenile pink salmon were sampled monthly in Prince William Sound and the Gulf of Alaska, and adults that survived to maturity were recovered at hatchery release sites the following year. Surviving fish exhibited significantly wider circuli spacing on the region of the scale formed during early marine residence than did juveniles collected at sea during their first ocean summer, indicating that marine survival after the first growing season was related to increases in early marine growth. At the same circuli, a significantly larger average scale radius for returning adults than for juveniles from the same hatchery would suggest that larger, fastergrowing juveniles had a higher survival rate and that significant size-selective mortality occurred after the juveniles were sampled. Growth patterns inferred from intercirculi spacing on scales varied among hatchery release groups, suggesting that density-dependent processes differed among release groups and occurred across Prince William Sound and the coastal Gulf of Alaska. These observations support other studies that have found that larger, faster-growing fish are more likely to survive until maturity.

Interannual Variability in Early Marine Growth, Size‐Selective Mortality, and Marine Survival for Prince William Sound Pink Salmon

“…This study extends the analysis by Moss et al (2005) to an interannual comparison of the timing and magnitude of differences in the growth and size of juveniles versus returning adults. Our main objective was to use scale patterns to compare the growth of juvenile pink salmon collected during July-September 2001 and that of surviving adults from the same year-class.…”

“…The first weeks at sea are thus often referred to as a ''critical period'' for growth in salmon because significant mortality occurs during this time (Parker Beamish and Mahnken 2001). Pink salmon are hypothesized to undergo a second critical period after the first growing season, presumably during the winter Mahnken 1999, 2001;Moss et al 2005). Under the ''critical size/critical period'' hypothesis, juvenile salmon that do not achieve a threshold size by the end of their first summer at sea will not survive the following winter (Beamish and Mahnken 2001).…”

“…Under the ''critical size/critical period'' hypothesis, juvenile salmon that do not achieve a threshold size by the end of their first summer at sea will not survive the following winter (Beamish and Mahnken 2001). Therefore, high growth rates during the first summer of marine life could be important in determining recruitment of a year-class (Beamish and Mahnken 2001;Beamish et al 2004;Moss et al 2005). Cross et al (2008) compared spatial and temporal patterns of pink salmon growth over the first 5 months of juvenile residence in Prince William Sound and in Gulf of Alaska coastal areas and showed that larger, fastergrowing fish experienced higher overall survival.…”

“…Moss et al (2005) used scales to compare the early marine growth of pink salmon entering Prince William Sound as juveniles in 2001 and returning in 2002 as adults. The authors showed that pink salmon that survived to adulthood grew faster than the average juvenile during the first summer at sea, and significant size-selective mortality occurred after the first growing season.…”

Stock‐Specific Size and Timing at Ocean Entry of Columbia River Juvenile Chinook Salmon and Steelhead: Implications for Early Ocean Growth

“…Several recent studies of Pacific salmon Oncorhynchus spp. have shown that individuals that survive to adulthood were often larger than average as juveniles (Beamish et al 2004;Zabel and Achord 2004;Moss et al 2005;Claiborne et al 2011; Thompson and Beauchamp 2014), and growth rates during initial marine residence are often correlated with survival in both Atlantic Salmon Salmo salar and Pacific salmon (Holtby et al 1990;Jonsson et al 2003;Miller et al 2014) and marine fish in general (Sogard 1997). However, while variation in timing and size at ocean entry is well documented between species or populations occupying independent river basins (e.g., Groot and Margolis 1991;Quinn 2005;Spence and Hall 2010), far less is known about the variation among populations within basins that enter the ocean at a common location (Beamish et al 2013).…”