Date of Award

5-2014

Degree Type

Masters Thesis

Degree Name

Master of Science (MS)

Department

Coastal Sciences, Gulf Coast Research Laboratory

Committee Chair

Chet Rakocinski

Committee Chair Department

Coastal Sciences, Gulf Coast Research Laboratory

Committee Member 2

Reginald Blaylock

Committee Member 2 Department

Coastal Sciences, Gulf Coast Research Laboratory

Committee Member 3

Phillip Lee

Committee Member 3 Department

Coastal Sciences, Gulf Coast Research Laboratory

Abstract

The Spotted Seatrout (Cynoscion nebulosus) is a euryhaline fish that inhabits coastal regions from Massachusetts to Texas. Due to its predaceous nature and proximity to coastal shorelines it is the most important marine finfish in the Gulf of Mexico. Because of this importance, Spotted Seatrout has been identified as a possible candidate for stock enhancement. Stock enhancement is the practice of releasing hatchery-reared juveniles to increase local recruitment beyond existing levels. However, raising enough Spotted Seatrout in an intensive aquaculture system has proven difficult due to aggression and cannibalism during the larval stages.

The manipulation of stocking density as well as feeding frequency might be used to reduce aggression and cannibalism. The goal of this study was to determine the effects of stocking density and feeding frequency on aggressive and cannibalistic behaviors in larval hatchery-reared Spotted Seatrout at both small-and production-scales. For the first two small-scale experiments, fish larvae were filmed and behaviors were quantified. Three different aggressive behaviors were quantified: nip (aggressor strikes prey causing prey to dart), chase (aggressor moves more than one body length toward prey), and capture (predator captures and holds prey but unable to consume). From these experiments, the author found that in all stocking densities (15, 30, and 60 fish L-1) there was a significant increase in aggression and cannibalism with time since feeding; and the author identified a density threshold of 30 fish L-1, beyond which the intensity of aggressive behaviors did not increase. These results implied that aggression in early stage hatchery-reared Spotted Seatrout might be alleviated by increasing feeding frequency, and furthermore, the Spotted Seatrout possibly could be cultured at densities higher than the current protocol allows.

From the second experiment, the author concluded that the 2 h feeding frequency treatment elicited fewer aggressive and cannibalistic acts than the 1 h, 4 h, and 8 h treatments. Lower levels of aggression and cannibalism associated with the 2 h feeding frequency treatment could be attributed to a balance between the optimal amount of live feed and the gut evacuation rate of larval Spotted Seatrout. In the third large-scale production experiment, I found a significantly lower per capita mortality and correspondingly higher specific growth rate for the 30 fish L-1 treatment than for the lower density 15 fish L-1 treatment. This difference resulted in a 45% higher production yield within the high density treatment.

Due to logistical constraints, only two replicates were available for the production experiment. Thus, the statistical power of this experiment was low; however, directional trends consistently suggested biological importance. Feeding every 2 h suggested an apparent decrease in per capita mortality, leading to a 15% increase in production. The observed higher specific growth in the high density treatment likely reflected a decrease in aggression. Also, size heterogeneity did not differ between density treatments, reinforcing the case that using a higher stocking density and a 2 h feeding frequency schedule should be ideal for the production of larval Spotted Seatrout. Overall, the results from the production-scale run were consistent with the results of the two earlier small-scale experiments, suggesting that a stocking density of 30 fish L-1 and a feeding frequency of 2 h should be effective for reducing aggression and cannibalism in the culture of Spotted Seatrout.

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