Graduation Year

2012

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Biology (Integrative Biology)

Major Professor

Susan S. Bell

Keywords

fish community, habitat value, mangroves, Poecilia latipinna, reproduction, tidal creek

Abstract

To assess the potential influence of coastal development on the quality of estuarine habitat for nekton, we characterized land use and the intensity of land development surrounding small tidal tributaries of Tampa Bay. Based on this characterization, we classified tributaries as undeveloped, industrial, urban or man-made (i.e., mosquito-control ditches). Over one-third (37%) of tributaries were determined to be heavily developed, while fewer than one-third (28%) remain relatively undeveloped. We then examined the nekton community from eleven tributaries in watersheds representing the defined land-use classes. Whereas mean nekton density and species richness were both independent of land use, nekton-community structure differed between non-urban (i.e., undeveloped, industrial, ditches) and urban tributaries. In urban tributaries, the community was skewed towards high densities of poeciliid fishes while typically dominant estuarine taxa were in low abundance or nearly absent. Densities of economically important taxa in urban creeks were also only half that observed in most non-urban creeks, but were similar to those observed in mosquito ditches. Furthermore, six of nine common taxa were found to be in relatively poor condition (6-22% smaller in mass), or were rarely collected, in urban creeks. Reproductive output was reduced for both sailfin mollies (i.e., fecundity) and grass shrimp (i.e., very low densities and few ovigerous females) in urban tributaries. Canonical correspondence analysis differentiated non-urban and urban tributaries based on greater impervious surface, less natural mangrove shoreline, higher frequency of hypoxia and lower, more variable salinities in urban tributaries. These characteristics explained 48% of the variation in nekton data, including the high densities of poeciliid fishes, greater energy reserves in sailfin mollies and low densities of several common nekton and economically important taxa from urban creeks. Our results suggest that urban development in coastal areas has the potential to alter the quality of habitat for nekton in small tidal tributaries as reflected by variation in nekton metrics between urban and non-urban tributaries.

To further evaluate the link between coastal development fish-habitat quality, we examined the relationship between landscape development intensity (LDI) and the body condition of juvenile sailfin mollies (Poecilia latipinna), a dominant forage fish in tidal tributaries. Morphometric condition, measured as least-square mean dry weight, did not differ statistically among tributaries (P = 0.85). In contrast, biochemical condition, measured as the concentration of triacylglycerol (TAG), the predominant storage lipid, was significantly different among tributaries (P < 0.0001). LDI explained less of the observed variation in TAG content (R2 = 0.18, P = 0.11) than long-term mean salinity (R2 = 0.81, P < 0.0001), which also tended to be lower in more intensively developed watersheds. We hypothesized that urban land use, characterized by considerably greater impervious surface than undeveloped lands, contributed to altered watershed hydrology, high freshwater runoff and low salinities in urbanized creeks. Together these factors appear to foster conditions conducive to lower energetic cost of osmoregulation in urban creeks, and development of a benthic microalgal community of greater nutritional value than the food resources available in non-urban tributaries. To our knowledge, this is one of the first studies to relate urbanization to the condition of resident fishes.

While not directly related to coastal urbanization, the level of predation risk within a habitat is a direct measure of habitat quality that could be reflected by the reproductive strategy of potential prey. To evaluate the use of reproductive metrics of fish-habitat quality, we examined reproduction in P. latipinna from eleven tidal tributaries. Our results revealed a gradient along which females produced many, small offspring at one extreme (mean=42 offspring, 17 mg each) and fewer, larger offspring at the other (24 offspring, 29 mg each). Reproductive allotment ranged from 14.9 - 21.5% maternal biomass. Based on our observation of divergent reproductive strategies, we experimentally tested the null hypothesis of no difference in predation risk among tributaries using a novel quantitative approach to estimate predation. We predicted greater risk in tributaries where mollies produced many, small offspring. Tethering confirmed increasing risk from 16.2 ± 5.3% SE to 54.7 ± 3.6% fish lost h-1 across sites in agreement with observed variation in reproduction. Predation was unexpectedly higher than predicted at one of the four sites suggesting that additional factors (e.g., food) had influenced reproduction there. Our results provide insight into the well-studied concept of predator-mediated variation in prey reproduction by quantitatively demonstrating differential risk for mollies exhibiting divergent reproductive strategies. While the observed range of variation in reproductive traits was consistent with previous studies reporting strong predator effects, higher than expected predation in one case may suggest that the prey response does not follow a continuous trajectory of incremental change with increasing predation risk, but may be better defined as a threshold beyond which a significant shift in reproductive strategy occurs.

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