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Winter energetics of young-of-the-year bluefish (Pomatomus saltatrix) : effects of ration and cohort of origin on survival

UNCW Author/Contributor (non-UNCW co-authors, if there are any, appear on document)
Joshua J. Slater (Creator)
Institution
The University of North Carolina Wilmington (UNCW )
Web Site: http://library.uncw.edu/
Advisor
Thomas Lankford

Abstract: The bluefish (Pomatomus saltatrix) population along the East Coast of the United States has experienced declines in both recruitment and adult abundance since the mid 1980s. At the end of their first growing season young-of-the-year (YOY) bluefish exhibit a bimodal length/frequency distribution consisting of larger, spring-spawned individuals (SP cohort) and smaller, summer-spawned individuals (SU cohort). While both SP and SU cohorts have been observed in the adult population in the past, recent studies have suggested that few SU-spawned individuals currently recruit to the adult stock. I investigated the hypothesis that the apparent recruitment failure of SU-spawned bluefish reflects negative size-selective overwinter mortality due to starvation. Due to mass allometries in energy storage and energy depletion, I predicted that larger, SP bluefish would 1) have greater energy stores prior to winter than smaller, SU bluefish, and 2) deplete their energy reserves at a slower rate than SU bluefish. Thus, I predicted that SP bluefish would exhibit greater overwinter survival (and therefore higher recruitment potential) than SU bluefish under starvation conditions. Overwinter mesocosm experiments performed at ambient temperatures were conducted to examine the effects of cohort of origin (SP versus SU) and feeding level (fed versus unfed) on the overwinter survival of YOY bluefish. Energetic condition (non-polar lipid and ash content) and survival duration of bluefish subjects were monitored over the 192-day experiment. SP-spawned bluefish possessed greater total lipid stores prior to winter than SUspawned individuals, and both cohorts relied on multiple tissue depots (liver, viscera, white muscle, red muscle and skin) for the storage and mobilization of lipids. When starved, SP and SU bluefish depleted their non-polar lipid reserves at similar rates over the first 31 days of the experiment. When food was present, both cohorts stored lipid at similar rates over the first 31 days of the experiment but depleted lipid reserves thereafter. This seasonal depletion pattern, despite the presence of food, indicates that lipid reserves are important for fueling routine metabolic requirements during winter and that bluefish may shift their energy allocation strategy from storage to mobilization/growth as winter progresses. When fed, both cohorts survived winter. When starved, SU bluefish began to exhibit starvation mortality six weeks prior to SP individuals. Although SU bluefish were more susceptible to overwinter starvation mortality than SP bluefish, their starvation endurance appears more than sufficient to permit overwinter survival under poor feeding conditions (>90% survival probability after 120 days without food and >60% after 150 days). Interestingly, SP bluefish suffered a brief mortality event during January when tank temperatures dropped below 6oC, suggesting that SP individuals may be less cold tolerant than smaller, SU individuals. Wild YOY bluefish sampled from inner continental shelf waters off North Carolina during winter did not approach critical energy levels as determined from starved laboratory bluefish. Given the high starvation endurance of SU-spawned YOY bluefish, I conducted a second winter experiment to assess the influence of forced activity and reduced prewinter lipid storage on their overwintering ability. It was hypothesized that high activity level and reduced pre-winter lipid storage would increase the vulnerability of SU individuals to winter starvation. The experimental design was a fully-crossed 2X2 factorial design with activity level (high versus low) and pre-winter lipid storage (high versus low) as factors. The high activity/low storage and low activity/high storage treatments were also tested in the presence and absence of winter food. Although the experiment was ended prematurely due to a system failure, lipid levels of bluefish at the time of death were quantified to examine whether the 2.5-month treatment exposures had measurable effects on bluefish energetics. Experimental results indicated that SU bluefish have a remarkable ability to store energy rapidly prior to winter. During a 30- day acclimation period SU bluefish were able to store more energy than was required to survive 2.5 months without food and at high (~0.8 body lengths sec-1) activity levels. Also, pre-winter lipid storage had a greater effect on bluefish energy reserves than activity level. Furthermore, SU-spawned YOY bluefish appeared capable of assimilating food in the winter, if available, allowing them to compensate for reduced pre-winter lipid storage. These observations are consistent with the defended energy level hypothesis. In conclusion, the remarkable starvation endurance ability of SU-spawned YOY bluefish, coupled with their capacity for rapid energy storage, and their ability to assimilate food during winter, indicates that SU bluefish are physiologically wellequipped to survive their first winter of life. These findings are consistent with recent energetics data reported for wild bluefish and do not support the overwinter starvation hypothesis as an explanation for the apparent recruitment failure of SU-spawned YOY bluefish.

Additional Information

Publication
Thesis
A Thesis Submitted to the University of North Carolina at Wilmington in Partial Fulfillment Of the Requirements for the Degree of Master of Science
Language: English
Date: 2009
Keywords
Bluefish--Atlantic Coast (U.S.), Bluefish--Life cycles--Atlantic Coast (U.S.), Marine fishes--Atlantic Coast (U.S.)
Subjects
Bluefish -- Atlantic Coast (U.S.)
Marine fishes -- Atlantic Coast (U.S.)
Bluefish -- Life cycles -- Atlantic Coast (U.S.)