The investigators will conduct a model-based investigation of the dynamics of a productive pelagic ecosystems in the Gulf of Maine. The middle trophic levels in highly productive marine ecosystems are typically dominated by a few species of pelagic fish, such as sardines and anchovies in upwelling environments or herring and/or capelin in temperate and subpolar regions. These species act as important conduits for energy to higher trophic levels, including larger fish, seabirds, and cetaceans. When abundant, small pelagics can exert significant pressure on their prey, typically large mesozooplankton. Small pelagic fish exhibit complex dynamics and managing these species under an ecosystem approach is challenging. This modeling study will track both the abundance and condition of representative copepods (Calanus finmarchicus, Centropages typicus), herring, and bluefin tuna. The investigators will use a rigorous comparison of conditions from the 1980s and 1990s to develop the model. They will examine the sensitivity of this ecosystem to changes in fishing pressure on the middle trophic levels and to changes in the magnitude and timing of primary production. They will also consider the impact of increased temperature on the ability of C. finmarchicus to accumulate lipids and alter the condition of herring and tuna.
The project will lead to improved knowledge of ecosystems with productive food webs. It will also directly impact address issues related to the management of the herring resource in the Gulf of Maine. The investigators will examine the consequences of ignoring condition of zooplankton and fish, as is the case with the current stock assessment. They will also explore the dynamical properties of the model ecosystem and consider under what conditions it is possible to have both abundant and well conditioned herring.
Oceans are a primary source of protein for people around the world. This fact makes rising ocean temperatures of particular concern as they are expected to significantly alter marine ecosystems and food webs by increasing metabolic rates, altering primary production, pushing the physiological limits of some species, and shifting distributions of other species. Pelagic forage fishes play critical roles in marine food webs by linking production from lower trophic levels (zooplankton) to higher trophic levels (larger piscivores such as tuna, striped bass, and marine mammals) and moving energy from feeding grounds to other ecosystems. The amount of energy moved across trophic levels or to new systems is directly linked to forage fish consumption rate. To better understand how climate change, commercial fishing, or other human impacts might affect marine systems, baseline estimates of forage fish consumption are needed. In the Gulf of Maine, Atlantic herring play pivotal ecological and economic roles by supporting the ecosystem and valuable commercial fisheries. In this study, we estimated the consumption rate of Atlantic herring in the Gulf of Maine to provide a baseline estimate for how this forage fish may transfer energy through the system. We first used a mercury-mass balance model to estimate annual consumption by herring, based on a diet equivalent to the calanoid copepod Calanus. Our estimates ranged from 51,408,900 MT in 2001 when the herring population was high to 7,064,800 MT in 2010 when the herring population was low. Interestingly, our modeling indicated herring consumption, when standardized to body size, increased with age, suggesting herring shift their diets or activity dramatically as they grow. Both hypotheses were supported when evaluated with an alternative model, but life history characteristics of herring suggest increased activity (i.e., migration) as herring grow is a more likely explanation for increased consumption (and energy demand) with size. Our results suggest highly-migratory fish species may have unique physiological requirements as they grow, and these should be considered when estimating prey consumption or energy demand of these species. This work supported the training of a Master’s of Science graduate student in quantitative modeling.
