Date of Award

Spring 5-9-2022

Document Type

Thesis: Open Access

Degree Name

MS Marine Science

Department

Environmental and Ocean Sciences

Committee Chair

Drew Talley

Committee Member

Mary Sue Lowery

Committee Member

Steven Searcy

Abstract

Gaining a deeper understanding of in-situ growth approximations for juvenile fishes is one way to understand how food consumption may affect fish growth. If variations in growth rate are strongly mirrored by past food status, then the condition (degree of well-being) of fish can be potentially used as a reference for relative prey availability. Furthermore, confirming that there is a positive relationship between food availability and fish growth rate is a critical first step to deepen our understanding of growth rate variation as well as examining if growth can be a proxy for habitat quality.

The California killifish (Fundulus parvipinnis) is an ideal test subject due to both their abundance and high site fidelity in Southern California marshes. This fish links intertidal and subtidal habitats by feeding on the vegetated marsh surface at high tide and returning to deeper marsh channels and seagrass beds at low tide. Tidal flooding of shallow creek channels allows this species to migrate into the vegetated marsh and forage for invertebrates. Thus, the California killifish is important in the estuarine food web, serving as a vector to transfer energy and nutrients off the marsh surface, and as a food source for higher trophic level species.

This controlled laboratory study aimed to validate daily otolith growth increments of the California killifish (Fundulus parvipinnis) and examine the reliability of three common growth measures: Fulton’s condition factor (Fulton’s K), RNA:DNA index, and recent otolith growth increments. Fulton’s condition factor is a morphometric condition index using length and weight measurementsto provide a direct measurement of growth. Quantification of nucleic acids in white muscle tissue characterized by the RNA:DNA index is another common indirect method for estimating growth rates and nutritional condition in marine organisms. Lastly, otolith increment analysis is an indirect method providing a permanent record of age and past growth rates. Using two different feeding levels in this study allowed closer comparison of how effectively these three measures of nutritional condition and growth can convey past feeding habits.

Fulton’s K values were strongly representative of past feeding rations after a two-week period, and food deprivation led to a considerable decrease in this index every week. RNA:DNA index values also showed significant differences between the feeding treatments. In contrast, there was weak evidence that recent otolith growth (avg. width of last 3 otolith increments) can relay food condition and the variable average daily otolith increment measurements were less responsive to feeding conditions. From our results, it appears that food consumption is reflected in daily otolith increments by a little over a one-week delay and the response to food was more clearly shown in normalized average otolith increment values. In addition, we recommend caution in the interpretation of otolith increments due to the variability of both the increment size and error introduced by the process of viewing otoliths. Ultimately, gaining perspective on how these growth indices change with respect to food rations will provide a more reliable interpretation of the health of California killifish populations, and potentially improve our assessment in how wetlands are functioning by viewing how well the fish populations are supported.

Comments

A big thank you for the California Killifish Association and University of San Diego for funding this research to make my idea into a reality.

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