Modelling energy sequestration and foraging by harbor porpoises in San Francisco Bay

The occupation of an ecosystem by a new top predator could affect the stability of a food web. In San Francisco Bay (SF Bay), there is evidence for a multi-decadal disappearance of harbor porpoises {Phocoena phocoena) that spanned from the 1940’s to the early 2000’s. Understanding the energetic role of this predator in the food web, in addition to the conditions that allowed for occupation of the Bay, is crucial to shedding light on the possible impacts of this species and on the state of the ecosystem. Here two modelling approaches were used in order to assess the energetics of harbor porpoises in SF Bay. The first model presented was an equation-based, velocity-dependent energy budget of harbor porpoises. As harbor porpoises have been shown to reproduce on an annual calving cycle in some regions, the minimum cost of transport was found for each possible reproductive state (non-pregnant, non-lactating (NPL)=1.6 J k g 'W ; pregnant (P)=1.8 J k g 'W ; lactating (L)=3.1 J k g 'W 1; and pregnant and lactating (PL)=3.2 J k g 'W 1). The total daily costs, in terms of carbon uptake, were then estimated for a NPL porpoise and a PL porpoise (NPL=341.3 g; PL=756.4 g) and when compared using a one-way ANOVA test, the difference was significant (F=15,797, d /= l, p<0.001). The second model presented was an agent-based, ecophysiological model of porpoise foraging in central SF Bay. Energy expenditure was estimated using the swimming speed and the reproductive status of the porpoise agent and energy intake was dependent on foraging success, measured by the capture of anchovy agents in SF Bay. The total daily energetic costs of each reproductive state were found (NPL=8060.4 kJ; P=8776.3 kJ; L= 19269.6 kJ; and PL=19985.6 kJ). The marginal value theorem was used as a test of habitat optimum for harbor porpoises in SF Bay and using the model, it was found that foraging success was more dependent on the number of anchovy schools than the number of anchovies per school. The number of schools that allowed for foraging levels higher than found for wild porpoises for a population of 50 animals foraging inside SF Bay was 41 schools of anchovies. When estimating total energy sequestration by differing numbers of harbor porpoises foraging in SF Bay, competition emerged as a factor influencing foraging success. For 33 and 165 porpoises foraging in SF Bay, total carbon consumed during a 6-hour foraging bout was estimated at 17,111.5 ±1 096.0 and 74,273.5 ± 8171.4 grams of carbon, respectively. As an upper-tropic level species that has recently increased its spatial habitat, it is crucial to investigate the possible impacts of harbor porpoises in San Francisco Bay and these two modelling platforms have allowed for the thorough investigation of the questions proposed.