Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Marine Science

Major Professor

Ernst Peebles, Ph.D.

Committee Member

Steve Murawski, Ph.D.

Committee Member

Linda Lombardi-Carlson, Ph.D.

Committee Member

Chris Stallings, Ph.D.

Committee Member

Brad Rosenheim, Ph.D.

Keywords

carbon isotope, fish diets, fish eye-lenses, fish movement, nitrogen isotope

Abstract

Data on the movement and diets of fish during a variety of life stages are important inputs to fisheries stock assessments and marine ecosystem models. Stable isotopes may provide previously inaccessible information on movement and diet of a variety of managed and forage fish species. Here I used several novel means of interpretation for stable isotope data to infer diets and movements of several important fisheries species over both short (weeks to months) and long (lifetime) timescales. To calculate a constant partitioning offset (CPO) between the δ15N of muscle and of liver tissue, I conducted a literature search of captive, diet-switch studies in marine teleosts. I found that δ15Nmuscle was uniformly higher than δ15Nliver across the literature. Mean (±SE) difference across all studies was δ15NM-L = 1.67 ± 0.14‰. I then compared the δ15NM-L values from 575 marine fishes representing eight species from continental shelf waters of Florida, USA to this CPO. I found that mean values of δ15NM-L for five of the eight species did not differ significantly from CPO, but mean values for three species were lower than CPO. These results suggest that the species at CPO were both stationary and consuming a diet of stationary prey. The species with δ15NM-L values far from CPO were either moving for consuming prey that was actively migrating through the area.

For lifetime-scale studies, I used the δ13C and δ15N values in the fish eye-lenses. I used the isotopic values in the eye-lens core (inner-most lamina) to estimate geographic location and habitat use of postlarvae from four species common to the northern West Florida Shelf (WFS). I found that isotopic values differed among the four species, suggesting different habitat by the postlarvae of each species. Relative isotopic values corresponded with relative geographies assessed through traditional fisheries demographic studies. In addition, correlation between eye-lens core isotope values and the parameters of eye-lens diameter or catch location each indicated that while some species move inshore during the postlarval period, others move little or move alongshore during this period.

I compared the eye-lens isotopic (δ13C and δ15N) profiles of two benthic-modifying species from the WFS to assess degree of habitat and diet stability in each species. Traditional fisheries data suggests that Tilefish (Lopholatilus chamaeleonticeps) shows high site fidelity and diet stability over the lifetime. Values of δ13C and δ15N in sequential eye-lens laminae were strongly correlated across the lifetime with increases in both profiles over time. These results suggest that Tilefish are highly stationary throughout the lifetime. Red Grouper (Epinephelus morio) are known for a sedentary lifestyle during the adult phase. However, as larvae and juveniles there is evidence that the species crosses the WFS to shallow water, and back to deeper areas. Eye-lens isotope profiles for adults showed low values of correlation between δ13C and δ15N. While δ15N increased steadily over the lifetime, δ13C did not. These results suggest much lower rates of both site fidelity and diet stability for Red Grouper than for Tilefish.

I used eye-lens stable isotope profiles combined with catch data and stomach content analysis to construct an in-depth depiction of Red Grouper diet and movement histories over the lifetime. I found that fish spawned on the WFS had isotopically distinct eye-lens cores from fish spawned in other regions. However, eye-lens core values were relatively consistent between juveniles and adults, suggesting a consistent region of postlarval habitat within the WFS system. Finally, I found that 60 to 66% of Red Grouper from WFS displayed a unique feature of the δ13C profile, whereby values peaked then decreased, completing the cycle before the end of the first year. Despite similarities in diet for individuals from the Florida Reef Tract and Campeche Bank, few had similar δ13C profiles. Combined with catch data and stomach content data, these isotopic data suggest ontogenetic cross-shelf movement along with a continually shifting diet over the first year of life.

The tools developed and tested here can provide additional data for interpreting movement and diets of important fisheries species for current single-species stock assessments. Using the difference between δ15N in the muscle and liver tissue of a single fish can provide information about the movement of fish over the preceding few months. Analyzing the δ13C and δ15N values in the eye-lens cores of individual fishes can indicate movement and diet during the earliest weeks of life using a historical approach. The whole eye-lens isotope profiles of δ13C and δ15N can suggest lifetime movement and combining the isotope profile data with traditional fisheries data may provide additional insight into the movement and diet of a species. These isotopic and statistical techniques will be invaluable as fisheries stock assessment continues to move to a more ecosystem-based approach.

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