It’s a standard assumption that the survival of adult fish is relatively constant through time and that most fluctuations in the number of adults come from variation in the number of young that are produced and survive to maturity.
However, new research with the black surfperch, a Pacific marine reef species, shows that changing food supplies and competition can alter survival of adult fish and be a major cause of fish populations fluctuating in abundance.
“More fish leads to less survival because they’re competing for limited prey resources.”
The analyses show that for black surfperch (Embiotoca jacksoni), survival of adults from year to year was strongly linked to both the amount of prey available and the number of fish sharing that food. The findings appear in the journal Ecology Letters.
“We found that the survival rate varies through time and is driven by local-scale processes of food variability and competition for that food supply,” says lead author Daniel Okamoto, a postdoctoral scholar at Simon Fraser University in British Columbia who earned his PhD at University of California, Santa Barbara. “That can have major implications for how we think about the impact of fishing.
“Our results demonstrate that mortality from fishing or other human activities can greatly amplify fluctuations in the number of fish in a population over time,” Okamoto adds. “This runs counter to all management and conservation goals for harvested species.”
Food really matters
The time series used in the synthesis came from the Santa Barbara Coastal Long Term Ecological Research (LTER) project at UC Santa Barbara. Professors Russell Schmitt and Sally Holbrook began surveying the food supply of surfperch on Santa Cruz Island in the early 1980s. Year after year, they returned to the same locations to survey the prey, their habitat, and the different age classes of fish.
“In our case, we wanted to know how much the dynamics of food supply influenced the population dynamics of a fish,” says Schmitt. “While everyone knows that food matters, we were surprised just how important it was compared to other well-known causes of variation in the number of adult fish on a reef.”
Longevity also makes the LTER data ideal for answering questions about fluctuations in species abundance over time. In addition, its data are designed to help understand the interactions among species. Okamoto used the data to perform sophisticated statistical modeling of the fish populations and examined interactions among the survival rate, the number of fish present, and abundance in their food.
“We’ve been able to pinpoint the factors that cause populations to vary without a lot of other confounding effects like fisheries,” Okamoto says. “We were able to find that there were strong interactions between the amount of food and the survival rate as well as the number of fish. More fish leads to less survival because they’re competing for limited prey resources.”
Unexpected ups and downs
Although the study targeted a single species, the research generalizes the results for other fish populations. “The degree to which survival rates vary through time is huge compared to what people generally tend to expect from fish populations in the ocean,” Okamoto says.
“Competition between adult fish tends to be a stabilizing force in populations because if there are too many fish, survival decreases and their numbers decline. And if there are too few, those fish do better and the population can grow.”
In fact, the researchers found that harvesting fish reduces the stability of that population by weakening the stabilizing force of competition, making the population much more susceptible to environmental variability.
“It’s known that species of fish that are harvested tend to fluctuate much more through time compared to species that are not fished, and scientists have speculated about what can cause fishing to amplify population fluctuations,” says Holbrook. “Our analyses provide a general mechanism by which added mortality from fishing or other sources can destabilize a fish population.”
Long-term studies like this, Okamoto notes, which examine the processes that give rise to variability, can affect how we think about sustainable fisheries or conservation measures going forward.
“We recommend greater investment in strategic monitoring of animal populations together with their food and natural enemies,” Schmitt says. “We also suggest adopting more precautionary approaches in fisheries until more is known about the complex relationships that shape their dynamics.”
The National Science Foundation, in support of the Santa Barbara Coastal LTER site, funded the work.
Source: UC Santa Barbara