Estimating target strength of estuarine pelagic fish assemblages using fisheries survey data.

In fisheries acoustics, the target strength (TS; dB re m2) is used to compute biological metrics such as fish biomass and density. The TS is challenging to characterize because of its stochastic relationship with fish physiology, orientation, depth, species assemblage, and size distributions. These challenges were addressed by using acoustic and physical samples of fish from trawls in the Penobscot River Estuary, Maine. The pelagic species assemblage was dominated by clupeids and osmerids. The TS was measured from individual fish using single target detection and echo tracking algorithms. An expectation-maximization algorithm was applied to identify the components of the TS and total length (TL; cm) distributions for the mixed species assemblages. Regressions were used to estimate the parameters of TS = α log10(TL) + ß. The parameters, α = 31.2 [standard error (SE) 0.87] and ß = -79.6 (SE 0.93), were similar to published studies from these species, but our slope and intercept were higher than those in studies from freshwater and lower than those from marine systems. These results suggest that acoustic surveys in estuaries with mixed species assemblages should carefully consider alternatives to "standard" TS-fish length equations. These results will provide necessary parameters to allow for interpretation of acoustic survey data from systems with a similar composition of pelagic species.

Effects of climate change on coastal ecosystem food webs: Implications for aquaculture.

Coastal ecosystems provide important ecosystem services for millions of people. Climate change is modifying coastal ecosystem food web structure and function and threatens these essential ecosystem services. We used a combination of two new and one existing ecosystem food web models and altered scenarios that are possible with climate change to quantify the impacts of climate change on ecosystem stability in three coastal bays in Maine, United States. We also examined the impact of climate change on bivalve fisheries and aquaculture. Our modeled scenarios explicitly considered the predicted effects of future climatic change and human intervention and included: 1) the influence of increased terrestrial dissolved organic carbon loading on phytoplankton biomass; 2) benthic community change driven by synergisms between climate change, historical overfishing, and increased species invasion; and 3) altered trophic level energy transfer driven by ocean warming and acidification. The effects of climate change strongly negatively influenced ecosystem energy flow and ecosystem stability and negatively affected modeled bivalve carrying capacity in each of our models along the Maine coast of the eastern United States. Our results suggest that the interconnected nature of ecosystem food webs make them extremely vulnerable to synergistic effects of climate change. To better inform fisheries and aquaculture management, the effects of climate change must be explicitly incorporated.

Characterizing seston in the Penobscot River Estuary.

The Penobscot River Estuary is an important system for diadromous fish in the Northeast United States of American (USA), in part because it is home to the largest remnant population of Atlantic salmon, Salmo salar, in the country. Little is known about the chemical and biological characteristics of seston in the Penobscot River Estuary. This study used estuarine transects to characterize the seston during the spring when river discharge is high and diadromous fish migration peaks in the Penobscot River Estuary. To characterize the seston, samples were taken in spring 2015 for phytoplankton identification, total suspended matter (TSM), percent organic TSM, chlorophyll a, particle size (2 µm-180 µm), particulate carbon and nitrogen concentrations, and stable carbon and nitrogen isotopes. The estuarine profiles indicate that TSM behaved non-conservatively with a net gain in the estuary. As phytoplankton constituted only 1/1000 of the particles, the non-conservative behavior of TSM observed in the estuary was most likely not attributable to phytoplankton. Particulate carbon and nitrogen ratios and stable isotope signals indicate a strong terrestrial, allochthonous signal. The seston in the Penobscot River Estuary was dominated by non-detrital particles. During a short, two-week time period, Heterosigma akashiwo, a phytoplankton species toxic to finfish, also was detected in the estuary. A limited number of fish samples, taken after the 2015 Penobscot River Estuary bloom of H. akashiwo, indicated frequent pathological gill damage. The composition of seston, along with ichthyotoxic algae, suggest the need for further research into possible effects upon resident and migratory fish in the Penobscot River Estuary.