Overwintering ability of young-of-the-year bluefish Pomatomus saltatrix: effect of ration and cohort of origin on survival

Abstract

Bluefish Pomatomus saltatrix have experienced declines in recruitment and adult abundance along the US East Coast since the mid-1980s. At the onset of winter, young-of-the-year (YOY) bluefish exhibit a multimodal size distribution including larger, spring-spawned fish (spring cohort) and smaller, summer-spawned fish (summer cohort). Declines in the adult stock appear to coincide with declines in recruitment success of the summer cohort. We investigated the hypothesis that poor recruitment success of the summer cohort results from size-selective winter mortality. Winter mesocosm experiments were conducted to examine the effects of cohort of origin (spring vs. summer) and food availability (fed vs. unfed) on winter survival of YOY bluefish. Spring fish entered winter with significantly greater lipid reserves than summer fish. When fed, both cohorts stored lipids during late fall, depleted lipid reserves during winter, and experienced high overwinter survival. When starved, both cohorts mobilized lipids from multiple depots (liver, viscera, white muscle, red muscle, skin) and summer fish experienced starvation mortality ~6 wk prior to spring fish. Although summer fish were more susceptible to winter starvation than spring fish, their starvation endurance (> 90% survival probability after 120 d) appeared more than adequate to survive natural winter conditions. Interestingly, spring fish suffered a brief mortality event during January when water temperatures dropped briefly below 6°C, suggesting that larger individuals are less tolerant of acute cold stress. The remarkable starvation endurance of summer-spawned bluefish, coupled with their capacity for rapid lipid storage during fall and reduced rates of lipid depletion at low temperatures, implies that members of this cohort are physiologically well-equipped to survive their first winter of life. Our findings are inconsistent with the hypothesis that winter starvation accounts for decreased recruitment of the summer cohort to the western Atlantic stock.

Winter energy storage dynamics and cohort structure of young-of-the-year bluefish Pomatomus saltatrix off North Carolina

“…The unusual energy depletion patterns observed in our data are not likely to be an artifact of sampling different portions of the population each month, because similar energy depletion patterns were observed in a laboratory experiment when fish were fed (Slater et al 2007). Slater et al (2007) determined that while larger bluefish started the winter with more energy reserves, by early spring there was no difference in energy reserves between cohorts when food was provided.…”

“…This may be due to increasing tolerance of acute cold stress with decreasing length. Slater et al (2007) held YOY bluefish in outdoor tanks at ambient temperatures during the winter and found Cohort 2 to be more tolerant of acute cold stress mortality than Cohort 1. Increasing cold tolerance with decreasing length has also been shown for Atlantic croaker (Lankford & Targett 2001).…”

“…The accumulation and subsequent depletion of energy stores in both muscle and liver tissue suggest both are important storage depots. Laboratory evidence also suggests white muscle and liver as important storage depots, along with viscera, red muscle, and skin (Slater et al 2007). The seasonal patterns in energy storage are strongly affected by changes in lipid content, due to its high-energy value and seasonal variability.…”

“…Slater et al (2007) determined that while larger bluefish started the winter with more energy reserves, by early spring there was no difference in energy reserves between cohorts when food was provided. When starved, bluefish depleted energy at similar rates regardless of size (Slater et al 2007). …”

Differences in juvenile trophic niche for two coastal fish species that use marine and estuarine nursery habitats

“…Also, due to the lack of previously published studies that report summer lipid values for age-0 bluefish, we were unable to compare the cohort-specific C:N lipid proxy patterns that we observed to values from the same month(s) from other regions or years. Still, it is worth noting recent work by Slater et al (2007) and Morley et al (2007) that found that spring and summer cohorts of age-0 bluefish manifested different energy allocation strategies by fall; members of the spring-spawned cohort entered the October-November overwintering period with higher lipid reserves than summerspawned fish that presumably shunted more energy into somatic growth. We observed the opposite pattern, i.e.…”