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

Abstract

The ecology of overwintering young-of-the-year bluefish Pomatomus saltatrix off North Carolina, USA, was examined for the 2001 and 2002 year-classes, to test the hypothesis that overwinter mortality affects the recruitment of summer-spawned bluefish. A trawling survey was conducted in Onslow Bay, North Carolina, from October 2001 to May 2002 and from September 2002 to June 2003 to determine bluefish abundance, cohort structure, energy density of white muscle and liver, and gut fullness. Up to 4 transects ranging from 0.4 to 16.1 km from shore were sampled monthly. Abundance of bluefish in Onslow Bay was high during the fall and declined with decreasing temperature in both years. Winter abundance was related to winter severity, with higher catches during the more mild winter of 2001 to 2002. At least 3 young-of-the-year cohorts were observed in both years. Gut fullness values generally followed temperature patterns, with reduced feeding during the winter. Energy reserves in white muscle and liver tissues peaked in November with larger fish having disproportionately more energy. However, by mid-winter there was little difference in energy reserves between the cohorts. These data suggest that larger fish deplete a greater portion of their energy stores as the season progresses while smaller fish deplete energy more slowly. Catch data show that summer-spawned bluefish survive the winter, but the magnitude of overwinter mortality remains uncertain.

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

“…On 19 November 2001, tanks were stocked with 15 SP or SU bluefish (n = 3 replicate tanks per treatment combination). Cohort assignment was based on bimodal length/frequency distributions of YOY bluefish collected in NC during fall 2001 (Morley et al 2007). Mean (± SD) initial fork lengths (FL) of SP and SU bluefish were 264 ± 16.6 mm and 206 ± 12.0 mm, respectively (Student's t-test, p < 0.01).…”

“…Wild bluefish for this analysis were trawled from continental shelf (5 to 20 m depth) habitats in Onslow Bay, North Carolina, during February and May 2002 (Morley et al 2007). Condition indices included liver dry mass, liver lipid and ash content, WM lipid density, WM ash content and total wet body mass.…”

“…While cold (<15°C) temperatures occurred for bluefish subjects during January and February, it is surprising that lipid reserves remained virtually depleted in late May. Water temperatures in the mesocosm tanks during April and May (>15°C) were within preferred ranges of bluefish and presumably suitable for feeding and energy storage (Lund & Maltezos 1970, Morley et al 2007. It is noteworthy that similar temperatures experienced by bluefish subjects during the previous fall were associated with rapid lipid storage by both cohorts.…”

“…This phenomenon would give the impression of the summer cohort's disappearance despite their presence in the adult population. Laboratory and field observations of juvenile bluefish growth do not support this explanation (McBride et al 1993, Buckel et al 1998, Morley et al 2007). Third, the summer cohort may recruit to localities outside the MAB, i.e.…”

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.…”