Curtis Brown

Date of Award

12-2007

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Marine Biology

Advisor

Richard A. Wahle

Second Committee Member

Robert S. Steneck

Third Committee Member

James M. Acheson

Abstract

Our understanding of the spatial and temporal variability in the strength of predator-prey interactions and their demographic effects are poor for subtidal marine taxa largely because of the small scale and short term of most ecological experiments. In this thesis I investigated how the interaction between the American lobster, Homarus americanus, and its predators varies over New England's biogeographic transition zone, a region encompassing one of the steepest latitudinal gradients in sea surface temperature on earth. For this analysis I conducted experiments and participated in surveys between 2003 and 2005, and for the temporal comparison used data from previous surveys and experiments conducted in 2000. Relative predation rates were measured with tethering experiments. Predators were directly observed through video surveillance and diver observation. Greater detail on the distribution and abundance of potential predators was provided by supplemental video surveillance and remote operated vehicle (ROV) during targeted dive surveys and research cruises, as well as state nearshore trawl surveys. Chapter 1 describes tethering experiments conducted in 2000 that revealed a significant gradient in predation pressure along the New England coast. Mortality rates of tethered lobsters were significantly higher in Rhode Island than in mid-coast Maine. Subsequent survey data indicate a corresponding gradient in the abundance and diversity of predatory fishes. Chapter 2 describes experiments repeated in the same regions during 2004 and 2005 while expanding our geographic coverage to include a third region in the eastern Gulf of Maine. A comparison of tethering experiments conducted in 2000 to those done in 2004 and 2005 indicated a decline in predation rates on juvenile lobsters in Rhode Island and an increase in Maine. Video monitoring indicated that visitation and attack rates offish had declined in Rhode Island, while attacks by crabs (Cancer irroratus and C. borealis) had become a significant agent of mortality in Maine. Surveys suggest a recent recruitment pulse of crabs has elevated mortality risk for juvenile lobsters in the Gulf of Maine. The recent appearance of crabs as significant predators of tethered lobsters raised the question of whether tethering increased the vulnerability of lobsters more to crabs than to fish. If so, this would introduce an experimental artifact that could lead us to overstate the importance of crabs as predators of juvenile lobsters in Maine. To evaluate this possibility, we conducted lab experiments to compare fish and crabs with respect to their ability to subdue . lobster prey, both on and off tether. Laboratory results suggest that while fish predation is greater during daylight hours, crab predation is greater at night. Taken together these results may have important implications for fishery models and management plans that assume spatially invariant rates of natural mortality. This may be especially true for species spanning ecologically heterogeneous regions and biogeographic transition zones.

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