Date of Award

2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Specialization

Biological Oceanography

Department

Oceanography

First Advisor

Candace Oviatt

Abstract

The American lobster (Homarus americanus) in southern New England sustained a particularly prosperous fishery during the 1980s and 1990s. However, in 2001 lobsters in the region began to decline, leading to the eventual collapse of the fishery in Rhode Island. The Narragansett Bay was no exception to this trend, with the number of lobsters caught by the University of Rhode Island Graduate School of Oceanography weekly fish trawl survey peaking in the 1990s before the decline. Today, few lobsters are caught in the Bay.

This dissertation seeks to identify whether environmental stressors and predators drive lobster populations in the Narragansett Bay, with a particular focus on the early life stages of lobsters. The first chapter displays the negative effects of warming bottom water temperatures on the growth early juvenile lobsters. Past research has largely focused on the effects of temperatures on larval and adult lobsters, but my study aimed to identify what postlarval lobsters experience in the Narragansett Bay. Both postlarval and one-year-old lobsters were raised for 12 weeks in one of three conditions, simulating either current bottom water temperatures in the Narragansett Bay (ambient), northern New England (3C cooler than ambient), or future temperatures under climate change (3C warmer than ambient). Temperature did not have a significant impact on the survival of lobsters. However, postlarval lobsters grew significantly slower and gained significantly less weight in warmer treatments. One-year-old lobsters all gained significantly less weight in the warm treatment than the ambient or chilled and molted less frequently than lobsters in the chilled treatment. Current Bay temperatures are not high enough to cause increased mortality, however a stunted growth at ambient temperature indicates that temperature is causing some physiological stress.

The second chapter demonstrates the threat of predation from scup (Stenotomus chrysops) and black sea bass (Centropistis striata) on juvenile lobsters. Both species have increased in the Bay as waters have warmed. Scup are not only more numerous but reside in the Bay for a longer period of time, whereas black sea bass have extended their range north and are the subject of frequent complaints by fishers who say the fish are caught in lobster traps. Two mesocosm experiments were performed at URI GSO’s mesocosm facilities. In each experiment, early juvenile lobsters were placed in a mesocosm with boxes of cobble to provide equivalent shelter to what is available in Narragansett Bay. Mesocosms were assigned as either a control (no predators), low predation (including either 2 scup or Asian shore crabs), or high predation (containing either 4 scup or 2 black sea bass). Significantly fewer lobsters survived in treatments with any number of scup or black sea bass, while Asian shore crabs had no effect on lobster survival. As warm water, migratory fish increase in number and residency time in the Bay, juvenile lobsters experience greater predation threats at a vulnerable stage, likely decreasing fishery recruitment.

The final chapter of the dissertation provides the first assessment of larval supply to the Narragansett Bay in decades and clarifies the timeline for the decline of lobsters in Rhode Island waters. Lobsters in the Narragansett Bay appear to recruit from larvae derived from coastal lobsters and are a minimal source of in-Bay recruitment. Larval sampling indicated that larval supply within and to the Bay is very low, and that larvae are most concentrated in the lower East Passage. Larvae often co-occurred with the copepod Anomalocera opalus, a coastal species that is rarely found inshore, indicating that larvae were associated with a coastal intrusion of Sound water into the Bay. Change point analysis showed that the decline of Rhode Island lobsters started in coastal waters, leading to a decline in lobster settlers before the number of lobsters caught in trawl surveys in the Narragansett Bay declined. Between 2001 and 2007, federal trawl surveys and landings data indicated that adult lobsters in coastal Rhode Island dropped, but juvenile settlement indices remained high. However after the decline of Rhode Island lobster landings in 2007, settlement indices for young-of-the-year lobsters dropped, indicating that larval supply from coastal lobsters had ceased. After this, the number of lobsters caught in the GSO Fish Trawl Survey at Fox Island was the last group to decline in 2010. Lobsters caught at Fox Island are usually young and small, just below legal catch size, and it is likely that their decline is the result of the decline of lobster settlers in the Bay, not the cause. Although temperature conditions in the Narragansett Bay now can be physiologically stressful for adult lobsters, it may have historically served best as a nursery. Larvae could be transported to the mouth of the Bay from coastal spawning, settle into the cobble, and then remain in the Bay as adolescents until they were mature enough to seek out preferable habitats in deeper waters offshore.

By examining multiple life stages, environmental stressors, and surveys for both lobster and cancer crab, this dissertation undertakes an in-depth analysis of the ecology and timeline of the decline of lobsters in the Narragansett Bay. The Bay, in its current condition, is stressful for both adult and larval lobsters, and the path to fishery recruitment is a gauntlet. Currently few larval lobsters are transported to the Narragansett Bay either from within Bay adult populations or from coastal spawning. This dissertation breaks new ground by clarifying the timeline for the decline in the southern New England lobster population, moving from offshore and coastal lobsters to in-Bay populations by severing the connection of the two groups through larval supply. However, the few larvae that do enter the Bay have to contend with physiologically taxing water temperatures, which stunt growth and make the path to recruitment longer. As they take longer to grow, juveniles are left more susceptible to predation from a booming host of warm-water fish. Few are likely to survive to recruitment and reproduction. Underlying each examined environmental variable is climate, which changes water temperatures, circulation patterns, and species overlaps.

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