Modeling at UCSB

Predicting dispersal of young fish

When resource managers set limits for a fishery, they typically rely on predicted numbers of young fish entering the population. However, collaborative research at UCSB shows that precise predictions of fish population replenishment may be difficult due to the complex, unpredictable nature of currents that transport the newborn fish.

The researchers created a model based on realistic ocean currents and fish behavior to forecast the travels of drifting young fish and invertebrates along the west coast. A grant from the Biocomplexity in the Environment program of the National Science Foundation supported the collaborative research. PISCO played a role in bringing together a broad range of experts on marine ecosystem dynamics and supplying biological data.

In the model, fish are born and released each day into the water at different points along the coast. The computer keeps track of where the young go in the complex three-dimensional motion of the water. After 20 days, the young in the model can settle and become adults, if they have arrived in a suitable coastal habitat. If they do not settle within 40 days, they die. A striking feature of the model is that the young settle in unpredictable pulses along the coast. At certain times and places, many young settle; at others, young fish are scarce. The young are able to settle when currents carry them toward the shore. But if currents flow offshore, the young are carried away from suitable habitat and are likely to perish before they can settle. The intermittent, patchy settlement of young has important implications for fishery management. For example, long-term monitoring of survival and growth of young fish may be needed to understand population trends in an area.

Modeling efforts at UCSB are conducted in conjunction with the McWilliams ROMS Group at UCLA. These efforts have played a critical role in the establishment of marine reserves in Southern California. Currently, the PISCO -UCSB group is working to incorporate biological parameters to this oceanographic model in order to assess the biological, environmental, economic, and social impacts of anthropogenic impacts in the coastal environment in collaboration with the Flow, Fish, and Fishing (F³) Group.

Larval dispersal is driven by mean currents, wind-driven Ekman circulation and coastal eddy motions as modified by the larval development time course and larval movements. In contemporary marine ecology, most models oversimplify the processes of larval dispersal and often describe it as a simple advection-diffusion process. The UCSB group assessed the roles of time-varying circulation on larval dispersal using coastal circulation simulations of the Southern California Bight (Dong et al. [1]; Mitarai et al. [2]). The results suggest that larval dispersal is driven primarily by coastal eddy motions (Fig. 1). Connectivity patterns for a single realization are highly variable because of intrinsic eddy-driven transport and variability in the winds. Mean dispersal patterns from a single release site show strong dependencies on particle-release location, season and year, reﬂecting annual and interannual circulation patterns in the Southern California Bight (SCB).

Figure 1. Sample particle trajectories from a single site. These particles are released in the nearshore of San Nicholas Island (yellow circles; site 130) on a) January 1, 1996, b) January 16, 1996, c) January 31, 1996 and d) February 15, 1996, and are transported passively by the simulated ﬂow ﬁelds. The blue lines indicate simulated 30-day trajectories. The red dots indicate the particle locations 30 days after the release.

In general, mean connectivity patterns are variable and asymmetric; mainland sites are good sources while both the Northern and Southern Channel Islands are poor sources, although they receive substantial ﬂuxes of water parcels from the mainland. These variable larval distributions (or connectivity patterns) can have important consequences in stock dynamics and community structure of nearshore marine populations, and can be a dominant mechanism structuring biogeography of marine organisms. The predicted connectivity gives useful information to ecological and other applications for the SCB (e.g., designing marine protected areas and predicting the impact of a pollution event) and provides a path for quantifying nearshore site connectivity using high-resolution numerical solutions of coastal ocean circulation. These results give new insights into the nature of larval dispersal and its potential for regulating population dynamics of nearshore marine species in the coastal ocean.

Figure 2: a) Nearshore sites and b) connectivity matrix linking the nearshore sites in the SCB via advection using the ROMS simulations. Each site has a 5-km. Here, the connectivity matrix quantifies the degree of inter-site connectivity for a PLD of 30 days. The solid black lines in panel b divide the nearshore sites into three regions: mainland (i, j = 1, …, 62), Northern Channel Islands (i, j = 63, …, 96) and Southern Channel Islands (i, j = 97, …, 135).

Dong, C., Idica, E.Y., and McWilliams, J.C., Circulation and multiple scale variability in Southern California Bight, Progress in Oceanography (to appear)
Mitarai, S., Siegel, D.A., Watson, J.R., Dong, C., and McWilliams J.C., Quantifying coastal connectivity in the coastal ocean: with application to the Southern California Bight, Journal of Geophysical Research - Oceans (in review)
Watson, J.R., S. Mitarai, D. A. Siegel, J. E. Caselle, C. Dong, and J. C. McWilliams. 2010. Realized and potential larval connectivity in the Southern California Bight. Marine Ecology Progress Series 401: 31–48.
J.R. Watson, C.G. Hays, P.T. Raimondi, S. Mitarai, C. Dong, J.C. McWilliams, C.A. Blanchette, J. E. Caselle, D.A. Siegel. Currents connecting communities: nearshore community similarity and ocean circulation. Submitted to Ecology.