Date of Award

12-2011

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Marine Biology

Advisor

Richard Wahle

Second Committee Member

Yong Chen

Third Committee Member

Andrew Pershing

Abstract

Understanding age-to-body size relationships and their variability in the American lobster is critical to our ability to assess the impact of harvesting on yield, as well as to forecast trends in future recruitment. Crustaceans lack conspicuous age markers and are ectothermic, therefore estimating growth and size-at-age is especially challenging. Because the American lobster’s geographic range spans one of the steepest north-to-south gradients in ocean temperature on earth, variability due to environmental factors is especially important to consider when modeling growth. To date, the effects of temperature on lobster biological rates, particularly growth, have not been incorporated into growth models used by stock assessments. In thisstudyI developed a step-wise growth model for three oceanographically contrasting regions: southern New England, Gulf of Maine, and the Bay of Fundy. These regions span a thermal gradient from a warm, summer-stratified regime in the south, to a cool, well-mixed regime in the north. In Chapter 1, regionally specified step-wise probabilistic growth models were developed from empirical juvenile size-frequency distributions and tagging data. In Chapter 2, I modified this model to incorporate temperaturein terms of growing degree-days, a method based on thermal requirements of growth. Both models provide regionally specified estimates of lobster size-at-age and its variability. Additionally, the growing degree-day model can predict how a changing climate would alter growth trajectories.

In Chapter 1 considerable regional differences in lobster growth were evident. In southern New England, growth is initially fastest, but an early onset of maturity slows growth dramatically at a relatively small size. In contrast, in the Gulf of Maine and Bay of Fundy, growth is initially slower than in the south, but maturity is delayed to a larger size and the subsequent decline in growth rate is less severe. The resulting regional growth curves give the mean and 95% confidence interval for the age lobsters recruit to the fishery.

The growing degree-day model described in Chapter 2 attempted to use temperature to explain regional differences in growth. Iftemperaturewas the dominant factor determining regional growth differences, I would expect the three regional growth trajectories to converge when expressed on a scale of growing degree-days. Such convergence was only partly realized. However, back-calculating these results to a scale ofcalendar-daysgave only slightly slower growth trajectories than the original model developed in Chapter 1. When using this model to make predictions for changing climate scenarios, varying the size-at-maturity along with temperature helped to explain regional growth trajectories. Future model development would benefit from an understanding of why the onset of maturity affects body growth more severely under warmer conditions.

Thus, while regionally-specified models have advanced our ability to account for regional differences in lobster growth, they have yet to adequately include the environmental factors that determine those differences. Temperature surely plays an important role in the observed regional differences in both the onset of sexual maturity and growth, however, I cannot rule out other factors that may also be important, such as food availability, population density, or local adaptation.

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