Santa Barbara Coastal LTER

Long-term Research Reveals Fish Re-distribution Trends

Reef communities along the California coast have changed substantially in recent decades and other major biological changes appear to be on the horizon. These changes are related to climate oscillations that produce abrupt regime shifts that typically occur every 20-35 years (Minobe 1997, 1999). Beginning in the late 1970s, mean seawater temperature and salinity shifted over a large area of the California Current causing the system to become warmer, fresher, and lower in dissolved nutrients (McGowan et al. 1998). This sudden environmental shift was accompanied by abrupt declines in ocean productivity. Long-term studies have enabled investigators at the SBC-LTER site to document ecosystem changes associated with this warming at the decadal scale. We have seen shifts to dominance by southern species in kelp forest fish at several sites in southern California, as well as dramatic changes in standing stocks of reef fishes and invertebrates at sites in the Santa Barbara Channel (Fig 5; Holbrook et al. 1997). Since the early 1970s, the proportion of species in fish assemblages that are cold-water, northern species has dropped by about half, while the proportion of southern, warm-water species has increased by nearly 50 percent. Overall, there has been a substantial decline in total fish abundance, which correlates closely with declines in productivity (Holbrook et al. 1997). The magnitude of declines has been similar for all species regardless of habitat, trophic level, mode of reproduction or level of exploitation (Brooks et al. 2002)

Figure 5. Changes in fish species composition in Southern CA. Kelp Forests

These patterns suggest an ongoing redistribution of marine species along the coast of California that is consistent with predicted northward shifts in species' ranges in response to ocean warming. Similar trends have been observed at other sampling locations in the southern California Bight, suggesting that this "redistribution" of species has resulted from regional declines in abundance rather than redistribution via the movement of individuals (Brooks et al. 2002). These shifts in reef fish assemblages have occurred gradually over the last 25 years, and they may have been undetected in typical three- to five-year research cycle.
The recent changes in ocean conditions have greatly affected the recreational and commercial fisheries in southern California (McGowan 1980). During warm climatic regimes, populations of cold-water, northern species get compressed into progressively smaller areas to the north, where fishing pressure becomes concentrated. Several of these once productive fisheries have virtually collapsed and fisheries managers have implemented drastic measures (e.g. large closures) to aid in their recovery.