SPATIAL ANALYSIS OF SPOT (LEIOSTOMUS XANTHURUS) POPULATION DYNAMICS

Abstract

Spot (Leiostomus xanthurus) is a forage fish that supports important commercial and recreational fisheries along the U.S. Atlantic coast. Assessment approaches to estimate abundance and mortality of spot for management on the U.S. Atlantic coast have previously not passed peer review. Our objectives were to (1) estimate spatial differences in biomass and mortality rates over time using fisheries-dependent indices and total mortality rates, and 2) estimate Spot abundance and fishing mortality rates in the Chesapeake Bay and the Atlantic coast using spatial population models. We calculated indices of biomass using commercial trip level data from gillnet fisheries and estimated total mortality rates using the Chapman-Robson approach with ageing data from fishery independent and dependent monitoring programs from Maryland to South Carolina. Dynamic factor analysis (DFA) was used to test for spatial differences in trends in biomass and mortality. We developed a two-region statistical catch-at-age model using the Template Model Builder package in R to distinguish the portion of the population along the Atlantic coast (New Jersey to Florida) from the portion in the Chesapeake Bay. We modeled ages 0-3+ for 2002-2019 with age-specific occupancy probabilities for each region. The data inputs for our model included multiple survey indices, commercial and recreational catch, shrimp trawl bycatch, and age composition of each data source. We implemented two models: one where fishing mortality varied between regions (SVM) and another where occupancy probabilities varied over time (TVO). Indices of biomass varied widely across space and time. Spot appeared to have high total annual mortality rates (>80% per year) that have increased in recent years. DFA indicated that stock biomass and mortality rates appeared to follow similar trends across regions. The TVO model was a better fit to the data and produced more biologically plausible estimates than the SVM model. In the TVO model, spot abundance was similar in both regions and has increased since 2015, which was a very poor year class. Patterns in fishing mortality differed between fleets with the commercial fishery declining substantially over time. The results from this work will aid in the development of an upcoming stock assessment for spot to inform management.