• Erica Petru

    Erica Petru
    University of WA
    Compared consumption rates by marine mammal species from ecosystems of a given size to fisheries consumption rates from those same ecosystems.

    My methods consisted of performing a literature search to find studies containing information about consumption rates by marine mammals and fisheries from specified study areas. I found 31 studies that lent themselves to the creation of 52 data points representing average consumption rates per species in a given ecosystem. The ecosystem sizes represented in these studies range from 55 km2 to 178,888,000 km2 and provide information about areas all around the world.

    A few sources of bias in this work are likely. One is a lack of random sampling; because the full body of information on this subject was not readily available to me, the data I used was limited to what I could find through the NOAA and University of Washington library systems. Furthermore, the studies available are likely biased towards more productive ecosystems which are generally of the most interest to fishers and fisheries researchers. Lastly, time is an important component in bioenergetics as systems are dynamic. To fully understand the relationship between consumption rates and ecosystem size, research covering greater spans of time would be essential.

    The graph that emerged from these studies shows a positive, linear slope. One probable contribution to the linearity of the slope is the energy equivalence rule, which states that energy consumption per unit area is independent of species body size.1 The thin black line in the graph represents the energy equivalence rule, which observes per area consumption rates of mammals in their geographic ranges. The thick red line is an interpolation between two extremes (local consumption in a one kilometer squared area derived from the energy equivalence rule and global consumption rates from all the world's oceans2). The difference in

  • Figure 1: Mean consumption rate by marine mammal species (blue diamonds) within an ecosystem in comparison to consumption rates by commercial fishing (red dots). The thin black line represents the expected relationship from the energy equivalence rule. The thick red line is an interpolation between two extremes (local consumption in a one kilometer squared area derived from the energy equivalence rule and global consumption rates from all the world's oceans).

    slope between the thin black line and thick red line shown in Figure 1 may involve the geographic ranges of the species in question: as the area of an ecosystem increases it becomes less likely that a species will inhabit the entire area, as the energy equivalence rule represents. That the points representing consumption rates are above the thick red line is likely a function of the bias towards productive areas. Regarding the comparison of fisheries consumption to marine mammal consumption, in all but two cases out of sixteen biomass removal by humans exceeded average biomass consumption by marine mammal species.

    Pending further analysis, the results may not only indicate whether there is a significant difference between consumption rates of marine mammals and those of humans, but they also could provide a very useful tool for the management of harvests from ecosystems for which managers have very little information. By identifying a relationship between consumption rates and ecosystem size, managers can estimate an appropriate harvest rate with only the knowledge of the ecosystem size. A precautionary approach to managing harvests

  • would restrict harvests to the lower limits of variability shown in these data.

    Other studies I have worked on this summer include reducing the dataset to marine mammals of human body size; evaluating the influence of primary production on this dataset; compiling diet composition information; evaluating consumption rates from single species and functional groups; and assessing variability in consumption rates between populations of the same species.

    1 Damuth, J.D. 2007. A macroevolutionary explanation for energy equivalence in the scaling of body size and population density. American Naturalist . 169:621-631.
    2 Tamura, T., and S. Ohsumi. 1999. Estimation of total food consumption by cetaceans in the world's oceans. The Institute of Cetacean Research, Tokyo, Japan.