Date of Award

2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Specialization

Biological Oceanography

Department

Oceanography

First Advisor

Jeremy S. Collie

Abstract

Two of the most ecologically and economically important species in the Mid- Atlantic Bight are Atlantic menhaden and the Atlantic striped bass. These species are important for human prosecuted fisheries, but these two species are also closely linked through ecosystem dynamics that are influenced by interactions between these species such as through predation. Beyond striped bass and menhaden, there are also other important species that are linked in this ecosystem. The focus of this dissertation is to estimate the trophic interactions among several important fish species in the Mid- Atlantic Bight, with an emphasis on the interactions between menhaden and striped bass. These species interactions are estimated by developing a multispecies, statistical catch-at-age model of a community of Mid-Atlantic species that explicitly quantifies the mortality due to predation.

Manuscript I: A model was developed for five important fish species in the Mid-Atlantic Bight. The model includes striped bass (Morone saxatilis), Atlantic menhaden (Brevoortia tyrannus), bluefish (Pomatomus saltatrix), weakfish (Cynoscion regalis), and scup (Stenotomus chrysops). The model was fit to commercial and recreational catch, survey, and diet data from 1985-2012. The model generated time and age varying natural mortality estimates, and allowed for the calculation of species-specific consumption estimates. Predation mortality increased on menhaden at all ages through the timeseries, peaking in the early and mid-2000s, and then declined. Predation mortality for scup was most notable on the first age class, peaked in the late 1990s and remained high for the remainder of the timeseries. The MSSCAA model performed close to the existing single-species models used for management and better than the multispecies virtual population analysis previously used for these species. The MSSCAA model provided a different view of some important biological information than did the single-species versions of the population models developed for comparisons, in particular with regard to fishing mortality, which was higher for menhaden.

Manuscript II: The model as developed for Manuscript 1 was used to project the five species populations forward under different management scenarios and in both long and medium term time periods. Under all scenarios investigated in the long-term projections, equilibrium levels were achieved by all of the species in the ecosystem. The assumptions going in to the projections were tested and indicated that natural mortality and recruitment are the most important considerations in the suite of assumptions tested. The projections also offer a different view of the population dynamics of the system when factoring in predation mortality. The standard constant natural mortality (M) approach to projections produces more optimistic outcomes than when the time and age-varying M is accounted for in the projection. This finding shows that accounting for additional dynamics in the ecosystem can add value to the current management process by giving managers a better sense of the structural uncertainty that exists around the various assumptions being made. If time and age varying natural mortality is the correct state of nature, this should be accounted for in the management action being undertaken.

Manuscript III: Prey-dependent interactions were investigated through the use of a two-species sub-model and projections. The two species used were menhaden and striped bass. The data indicated there may be a link between decreased consumption of menhaden by striped bass and increased natural mortality in the striped bass population. The investigation was extended to look at the impacts that this dynamic has on the two populations by programming the prey-dependent mortality effect into a multispecies estimation model. The parameters from the estimation model were then used to project the population to examine trade-offs that occur under a set of simple management strategies. The trade-offs were found to be important and indicate that there is an interaction between fishing mortality (F) and natural mortality (M) in both the medium and long-term projections. A main finding was that under a no-menhaden fishing scenario, the management outcomes were better for striped bass. This was true under different F strategies for striped bass, as the decreased natural mortality offset F. It is important to note that these management scenarios were developed to illustrate trade-offs and are not offered as actual management recommendations. Analyses like these can offer important information to managers by way of giving them more information to consider when developing measures that attempt to meet goals, as fishing impacts on one population can affect the attainment of goals on other populations.

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