Publication Date

2008-01-01

Availability

Open access

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Marine Biology and Fisheries (Marine)

Date of Defense

2008-06-26

First Committee Member

Dr. Joseph Serafy - Committee Co-Chair

Second Committee Member

Dr. David Die - Committee Co-Chair

Third Committee Member

Dr. Martin Grosell - Committee Member

Fourth Committee Member

Dr. Jerome Lorenz - Outside Committee Member

Abstract

Mangroves and seagrass beds serve as essential fish habitat for many economically- and ecologically-valuable species. Depending on their location, these shallow-water habitats are often characterized by substantial fluctuation in salinity levels, which can represent a source of osmoregulatory stress for associated organisms. In South Florida, one of the most important fish species that utilizes these habitats is the gray snapper (Lutjanus griseus). Although this species constitutes a significant portion of the region?s total recreational fishery harvest, the effects of salinity on its distribution, physiology and behavior remain poorly understood. The main goal of this thesis was then to investigate the ecophysiological basis of habitat selection by the gray snapper. Specific objectives include to: (1) examine patterns of distribution and abundance across gradients in environmental salinity; (2) measure physiological status and responses to controlled salinity challenges and; (3) conduct behavioral trials to examine for salinity preferenda (if any). To begin investigating if salinity could be a primary factor structuring the gray snapper assemblages, I examined empirical data collected from Biscayne Bay to test the null hypothesis that gray snapper abundances were evenly distributed along the full salinity range at which samples have been collected. Using the delta approach, three abundance metrics (frequency of occurrence, concentration and delta density) were used as an index for the distribution and abundance of this species. Results indicated that abundance patterns for the smaller gray snapper were consistent with a strategy of reducing osmoregulatory costs by selecting intermediate salinities. However, corresponding abundance patterns for subadult gray snapper were inconsistent with this strategy of minimizing energetic costs, suggesting that this life stage may be indifferent to the range of salinities at which they were observed. These patterns helped developed further hypotheses regarding the ecophysiology of juvenile and subadult gray snapper, the latter of which was then tested via laboratory experiments. Subsequently, I challenged fish in the laboratory with six different salinity treatments (0, 5, 30, 50, 60 and 70ppt, including control) for 192 consecutive hours and collected blood samples at different time points. Results indicated that physiological stress to salinity changes is unlikely to occur at a salinity range of 5 to 50 ppt. At salinities of 0 and 60 ppt transient significant changes in plasma osmolality and/or blood haematocrit were observed, but were corrected after an initial adjustment period of approximately 96 hours. At the highest salinity treatment (70 ppt), a constant osmolality could not be maintained, resulting in death for all fish within 48 hours of exposure. Overall, these findings demonstrate the strong euryhalinity and extraordinary tolerance of this species to both extreme hypo- and hypersaline environments. Finally, I investigated the salinity preference and effects on swimming behavior of the gray snapper in an automated salinity choice shuttlebox via 48-hr trials. In general, gray snapper tested displayed either one of two distinctively different salinity preferences. Half of gray snappers displayed a salinity preference in the range of 9-15 ppt, whereas the other half displayed a salinity preference in the range of 19-23 ppt. Recorded swimming speeds in all fish tested reflected a significant, but weak negative linear relationship with salinity during both time periods of the day (light and dark); however, gray snapper were usually most active during the dark period across all salinities. Overall, these findings reveal that gray snapper prefer slightly hyperosmotic salinities that may minimize the physiological costs of osmoregulation compared to extreme salinities.

Keywords

CERP; Avoidance Behavior; Lethal Limit; Acclimation; Ecology; Bioenergetics

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