Climate and Marine Populations
Our work on potential influences of climate change on marine populations began in the early 1990s with funding from NSF through the US GLOBEC program, part of the US Climate Change Initiative. Our first step was the definition of some of the characteristics of marine metapopulations in the context of Dungeness crab. This work on spatially distributed metapopulations led into some of the results focused on MPAs described above.
Another early step was to describe how variability in catches in salmon and crab fisheries in the California Current were primarily driven by warm vs. cool periods (i.e., ENSO vs. not ENSO, similar to PDO forcing).
Our GLOBEC research on the population dynamics of Pacific salmon focused on how the differences in age structure between coho and Chinook salmon would cause them to have different responses to a variable environment. Some believed that the narrow spawning age structure of coho salmon relative to that of Chinook salmon explained the greater variability seen in coho salmon (primarily a decline in the 1990s. We showed that the expected differences in response to the environment were not great enough to cause such a difference. This was lead by a postdoc, Forrest Hill.
We gained a quantum leap in understanding when we analyzed the response of age-structured populations with density-dependent recruitment to random variability in growth and survival rates. This analysis characterized that response in terms of frequency dependent sensitivity. We rediscovered the fact that such populations were more sensitive to environmental frequencies near 1/(generation time) and to low frequencies. This effect, called cohort resonance, had been discovered and described earlier by Bjornstadt, et al. in 2004. We showed that fishing caused caused an increase in the frequency-dependent selectivity, and also increased variance. This provided a possible mechanism underlying the observed increases in variability with fishing.
We used this stochastic age-structured modeling approach to show how salmon and cod populations would respond differently to changing frequency content of future environments.
We then showed that the cohort resonance phenomenon provided an explanation for the “cohort-dominant” cycles in sockeye salmon in the Fraser River and Bristol. The cause of these had been sought for a number of decades. Since not all sockeye salmon show these cycles, and populations can move in and out of the cycles, we determined the conditions under which these cycles were likely to occur: 1.the population being close to the point of collapse, 2. variability in survival, and 3. narrowness of the spawning age distribution. Will White, at the University of North Carolina, Wilmington (formerly a post doc at UC Davis) led this effort.
Next we compare the manifestation of the cohort resonance phenomenon in species of differing longevities, using coho salmon, Pacific hake and Pacific Ocean perch as examples. In the unfished state the frequency selectivity of cohort resonance is stronger in recruitment,egg production and catch of the shorter lived species. However, it increases with fishing more rapidly in the longer-lived species. This publication was put into a special issue of ICES Journal of Marine Science commemorating the 100th anniversary of Johann Hjort's classic pubilcation on the causes of variability in recruitment. We did this because Hjort had correctly identified the fundamental roles of nutrition and transport in recruitment, but he dismissed any effect if variability in adult abundance, and the resonance effect of cohort resonance is basically an example of an effect of adult variability. This paper was selected as an Editor's Choice in ICES JMS, and its impact is described on the home page of the journal.
