Award: OCE-1041731

Small silvery fish like herring, sardines, and anchovies are at the center of marine foodwebs. They are important prey for whales, birds, and larger fish like tuna and cod. They are also predators on zooplankton, and thus, they help channel the energy stored in small species up to the larger predators. The main goal of our project was to understand the role that small pelagic fish play in marine ecosystems, specifically how changes in their abundance and condition (fat content) influence and are influenced by the abundance and condition of their predators and prey. Our project focused on the interactions between herring, giant bluefin tuna, and the copepod Calanus finmarchicus in the Gulf of Maine. Our project was motivated by a substantial decline in the condition of bluefin tuna feeding in the Gulf of Maine. Previous work noted that tuna in the 1980s were heavier than tuna of the same size in the 1990s. The decline in tuna condition was perplexing, since the number of herring in the Gulf of Maine increased dramatically in the late 1990s. Thus, tuna were starving in a "sea of plenty." We hypothesized that the decline in tuna condition was due to a decline in the fat content of herring, which was in turn due to a decline in Calanus finmarchicus abundance in the 1990s. Calanus is one of the most abundant animals on Earth, and it can store a lot of fat in its rice grain-sized body. First, we examined time series of Calanus abundance in the Gulf of Maine and other regions in the northwest Atlantic. We found that declines in Calanus abundance occurred consistently when the surface waters of the ocean became warmer and/or fresher. This suggests that physical changes were behind the decline of Calanus in the Gulf of Maine, not the increase in the number of hungry herring. Next, we examined the condition (weight divided by length) of herring. We found that herring condition did not vary substantially, but that herring were on average smaller in the 1990s. Our work suggests that the decline in the mean size of herring was related to the abundance of herring (i.e. herring competing with one another) and also an increase in the number of younger (and smaller) herring. Finally, we turned to the tuna. We found that the decline in the mean size of herring was strongly associated with the decline in tuna condition. However, this doesnÆt explain why tuna would be skinnier—why wouldnÆt the number of herring make up for the fact that the herring were smaller? We built a mathematical model of how tuna select prey, and one of the predictions of the model is that tuna should be very sensitive to the proportion of large to small herring, not to the total abundance. This model explains why tuna were skinnier in the 1990s and also why they began leaving the Gulf of Maine for Canadian waters where herring were still large. Our work has implications for how we manage fish stocks. Herring and other small pelagic fish are an important resource for larger predators, and several authors have advocated for reducing fishing on herring in order to encourage the rebuilding of populations of larger fish and marine mammals. Our work suggests that managing for abundant herring might not benefit all predators equally. Species like humpback whales that can eat hundreds of herring at a time would certainly benefit, but species like bluefin tuna that eat them one at a time depend on having abundant larger, older herring. Last Modified: 01/07/2015 Submitted by: Andrew J Pershing