An investigation of effects of dissolved oxygen level, sediment type, stocking density and predation on the growth rate, survivorship, and burrowing behavior of juvenile brown and white shrimp.
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Date
2003-06-03
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Abstract
Brown shrimp (Farfantepenaeus aztecus) and white shrimp (Litopenaeus setiferus) are important components of estuarine food webs and support valuable aquaculture and commercial fishing enterprises. Both species exhibit clumped distributions correlated with sediment type. Escalating cultural eutrophication in estuaries has increased the frequency, spatial scale and duration of concurrent hypoxia (dissolved oxygen, DO, ≤ 2.0 ppm), which is often associated with specific regions of estuarine bottom, where shrimp may have higher risks of exposure with negative impacts. Shrimp densities may increase in unimpacted habitats due to avoidance of hypoxia, where increased predation rates, density-dependent mortality and growth attrition may result. I used laboratory and field experiments to investigate effects of DO level, sediment type, density and predation on the growth rate, survivorship, burrowing behavior and sediment preference of juvenile brown and white shrimp.
In the laboratory, white shrimp growth rate was positively related to DO level from 2-6 ppm, but brown shrimp growth rate was not. Fluctuating (super-saturated/hypoxic) DO reduced growth rate relative to constant high DO in white shrimp only. White shrimp appeared to display an acclimation response whereas brown shrimp did not.
In the laboratory, brown shrimp emergence was examined under different DO (high or low) and predation threat (caged predator, uncaged predator or no predator) treatments. DO level had a significant effect in all cases. With an uncaged predator, emergence at low DO was significantly less than with no predator, suggesting a trade-off between emergence at low DO and the probability of a lethal encounter.
In the laboratory, burrowing preferences of both brown and white shrimp among five sediment types (sand, peat, sandy mud, muddy sand, and shell sand) were significantly different from the expected null ratio (1:1:1:1:1). Brown shrimp frequencies were higher than expected in sand and peat, as expected in muddy sand, and lower than expected in sandy mud and shell sand. White shrimp frequencies were higher than expected in sand, as expected in sandy mud and peat, and lower than expected in muddy sand and shell sand.
Field mesocosm experiments revealed a significant negative relationship between shrimp density and growth from 5-20 shrimp⋅m-2, but there was no effect on survivorship. Sediment type (sand, peat, muddy sand) had no effect on growth rate or survivorship. Predator presence (1 or 2 predators⋅m-2) resulted in significantly lower survivorship than in the absence of a predator and demonstrated the efficiency of pinfish as shrimp predators.
Results indicate that there may be different consequences of sublethal low DO, even at levels that are not considered hypoxic, for the growth rates of these two closely related species, with white shrimp being more vulnerable. Hypoxia increases the vulnerability of shrimp to predators by increasing exposure time, but predation threat can dampen the emergence response, emphasizing behavioral considerations when examining the net effect of biotic and abiotic factors. Both species preferred sand substrate, suggesting that observed field distributions may be based less on foraging considerations than on other factors (substrate penetrability, ease of respiration when burrowed, habitat recognition, predatoravoidance tradeoffs, and on proficiency at locating preferred substrate). Substrate type did not affect shrimp growth rate, whereas shrimp density appears to be critical. Outside hypoxic zones, density-mediated growth attrition and predation mortality may be important factors affecting shrimp populations. This highlights the necessity for future research using largerscale studies to more precisely characterize and quantify the extent of such impacts.
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Keywords
predator, preference, prey, sediment, pinfish, Penaeus setiferus, Penaeus aztecus, Litopenaeus setiferus, Lagodon rhomboides, hypoxia, Farfantepenaeus aztecus, eutrophication, emergence
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Degree
PhD
Discipline
Zoology
