First off, some background. I'm doing my grad work at East Carolina University under the direction of Dr. Rulifson (whose spiny dogfish work can be seen at spinydogfish.org - check the Contact section for a certain handsome devil). I've been interested in sharks and shark research since I was but a wee lad, and as stated two posts down, I'm convinced that spiny dogfish are secretly some of the most interesting sharks out there. So here it is for your reading pleasure; the quick and dirty version of my thesis proposal. Keep in mind that this is written for a blog audience, so I've kept the technical aspects and scientific language to a minimum (I do have citations though). Since any good thesis lives or dies on feedback, questions and comments are encouraged.
Food and feeding of the spiny dogfish (Squalus acanthias) overwintering off the coast of North Carolina and the effects on the marine community
Introduction: Spiny dogfish have long had a reputation for being generally bad neighbors with fishermen, whether they're eating the catch, wrecking gear, or driving off other commercially-important species. This has lead to an interest among fishermen and scientists in the feeding habits and community interactions of these small sharks. Diet data is routinely collected by the NOAA/NMFS bottom trawl surveys, and this data has lead to a good pool of literature on the large-scale, ecosystem-wide feeding habits of dogfish (Bowman et al. 2000, Link et al. 2002). However, the sheer scope of this work has potentially had the effect of overshadowing localized short-term feeding events such as seasonal migrations and spawning runs, and the amount of predation going on during these events can be significant (Beamish et al. 1992).
Another aspect of dogfish migrations is that the sharks occupy the same feeding guilds as many of the other, similar-sized predatory fish that are targeted by both commercial and recreational fishermen (Garrison and Link 2000). This means that spiny dogfish potentially compete for food with a lot of the fish we like to eat, including striped bass, bluefish, cod, and flounder. It has long been suspected that the aggressiveness and sheer numbers of large migrating schools of dogfish have the effect of driving out other, more desirable species.
A large chunk of the dogfish population makes a seasonal migration south from New England for the winter, coming to a stop off of North Carolina (McMillan and Morse 1999). The purpose of my project is to provide data on two aspects of the large seasonal presence of spiny dogfish in North Carolina waters; feeding habits and the effect of dogfish presence on the relative abundance of other species in the area.
Methods: I'll break these up by the two main objectives of my study. Sampling of dogfish will be performed opportunistically aboard research trawl surveys passing through North Carolina waters during the months of January-March. I'm already due to be on the NOAA R/V Henry Bigelow (trip blogging, anyone?) when it comes through North Carolina waters, and am constantly in the process of getting extra trawl sampling in. I'm sampling by trawl because this seems to be the "least worst" method for getting stomach content data (hook and line sampling selects for hungry fish with empty stomachs, while the stress of capture in gillnets tends to cause dogfish to puke before being hauled aboard).
Feeding Habits: I'll be sampling dogfish stomach contents non-lethally using the stomach tube gastric lavage method (I'll be giving this awesomely gross method the full post it deserves at a later date). The gut contents will be preserved and brought back to the lab, where they will be identified as close to species level as possible. Hopefully enough of the stomach contents will be relatively intact to get accurate counts and weights, but at the very least I should be able to get presence/absence for any given prey category. If enough quantitative data is available, I'll figure out the overwinter consumption rate and compare it with fishery landings of prey species in North Carolina to see just how big a chunk dogfish may be eating out of the stocks. Also, the lavaged dogfish will be divided into groups based on size and sex to assess the affect of growth and sex on the diet.
Community Interaction: While on the trawl surveys I'll be taking down relative abundance data for all the trawls made within North Carolina waters and comparing the species composition in trawls containing dogfish with those lacking the marauding sharks. This is to see if the mere presence of dogfish may be altering the distribution of other species in the North Carolina marine community.
Expected Results: Right now all I've got for expected results is basically what the literature and common sense would suggest; prey species will probably be the most common forage fish available and will co-occur with dogfish often, while other predatory fish will likely show a low co-occurrence with them.
So that's the talking-points version of what I'll be putting my sweat, blood, heart, and soul into during my graduate career. As of right now the proposal defense is being put together and the committee is being summoned, and so far I've gotten reasonably positive feedback. I'll be updating my progress as progress is made, so stay tuned. By this time next month I should be knee-deep in dogfish puke.
Sources Cited and Further Reading
Beamish, R.J., B.L. Thompson, and G.A. McFarlane. 1992. Spiny dogfish predation on chinook and coho salmon and the potential effects on hatchery-produced salmon. Transactions of the American Fisheries Society 121:444-455.
Bowman, R.E., C.E. Stillwell, W.L. Micheals, and M.D. Grosslein. 2000. Food of Northwest Atlantic fishes and two common species of squid. NMFS-NE 155, 138p.
Garrison, L.P. and J.S. Link. 2000. Dietary guild structure of the fish community in the Northeast United States continental shelf ecosystem. Marine Ecology Progress Series 202:231-240.
Link, J.S., L.P. Garrison, and F.P. Almeida. 2002. Ecological interactions between elasmobranchs and groundfish on the Northeastern U.S. continental shelf. 1. Evaluating predation. North American Journal of Fisheries Management 22:550-562.
McMillan, D.G. and W.W. Morse. 1999. Essential fish habitat source document: Spiny dogfish Squalus acanthias, life history and habitat characteristics. NMFS-NE 150, pp. 1-3.