The group of researchers was evaluating the interaction after results from an earlier report found that fishing of forage species had a large effect on predator population, said the Marine Ingredients Organization (IFFO). Those harvested fish are used in several areas including as feed ingredients.
The new study was initiated because there were some questions regarding the methods used in the initial project, said Ray Hilborn, with the school of aquatic and fishery sciences at the University of Washington and corresponding author.
“When the original Lenfest [Forage Fish Task Force] report came out, a few of us said it seemed that the methods they were using were not up to the questions they were asking,” he told FeedNavigator. The report also offered several policy recommendations, he added.
“It was on our radar screen,” he said. “And one of the things I’ve been interested in looking at is the intensity of natural fluctuation in populations, and forage fish are notable for how much they vary naturally.”
The interaction between forage fish populations and predators is more complicated than may have been suggested by earlier studies tracking that relationship, and several factors need to be considered when analyzing the role that fishing plays on that relationship, he said. “The key point isn’t that there isn’t an impact, but that you have to argue case-by-case,” he added.
Several factors need to be considered when assessing the interaction among predators, forage species, and fishing of those forage species, the researchers said in their study. “We show that taking account of these factors generally tends to make the impact of fishing forage fish on their predators less than estimated from trophic models,” they added.
Factors that have to be considered when examining the effect of fishing from low trophic or forage species on higher level marine predators include:
- Natural variability of forage fish
- The weak relationship between spawning size of forage fish, recruitment and use of environmental productivity regimes
- Size distribution of forage fish and size selective predation
- And, the change in distribution of the forage fish as it influences the reproductive success of predators
The results from Hilborn’s group have seen responses from groups including IFFO.
Previous research based on models suggested that forage fish were more valuable when left in the ocean and recommended reducing forage fish collection rates by 50% to 80%, said IFFO. However, the new paper presents an argument for a more case-by-case basis for management.
“For fisheries management, such a precautionary approach would have a large impact on the productivity of forage fisheries,” the organization said. “As groups such as IFFO have noted, these stocks contribute strongly to global food security, as well as local and regional social and economic sustainability.”
It is important that fisheries are managed with an effort to balance requirements from the ecosystem, coastal communities and human nutrition, IFFO said. The new results provide additional guidance and update conclusions from past reports.
“It is also well-established that forage fisheries provide substantial health benefits to human populations through the supply of long-chain omega-3 fatty acids, both directly through consumption in the form of fish oil capsules, and indirectly through animal feed for farmed fish and land animals,” the organization said.
Fishing of low trophic or forage fish has generated interest in recent years, the researchers said. These fish include small pelagic fish, squid and juveniles of many species.
The evidence and theory suggest that fishing can limit the abundance of some fish stocks and can affect predators’ reproductive success by the density of their prey, they said.
“Although it would therefore seem obvious that fishing forage fish would have a negative effect on the abundance of their predators, the empirical relationships between forage fish abundance and predator abundance, or population rates of change, have not been examined in a systematic way,” they said.
In the study, the group examined 11 species of forage fish in the US, including what animals eat them and the role the species play in their predators’ diets, they said.
Species’ predators were identified, estimated fish abundance was analyzed and several models were fit to the data being assessed, they said. A simulation model also used information from fisheries regarding six different species of forage fish to evaluate the potential reduction in food for predators given a 5,000-year timespan.
“The question that they were asking is an important question, but to ask it properly you need to have analysis that includes the important biology,” said Hilborn of earlier evaluations. “We’re just doing a more detailed look at the biology, which you need to do to understand fishing forage fish and what happens to their predators.”
The goal of the study was to identify the key factors that should be considered by analyzing the effect of fishing on forge fish, said the researchers. The group found, overall, that the models previously used were “frequently inadequate” for determining the role the fishing of forage fish plays on their predators.
“The most important feature that needs to be considered is the natural variability in forage fish population size,” they said. “Their abundance is highly variable even in the absence of fishing, and creditable analysis of the fishing impacts must consider how the extent of fishing-induced depletion compares with that of the natural variability.”
The research results did offer some unexpected results, said Hilborn.
“I was really surprised that we didn’t see any empirical data showing the relationship between predators and prey,” he said. “We only looked at American fisheries, but didn’t find at any correlation with fish and the predators.”
The majority of cases did not offer an obvious relationship between prey and predator abundance, the researchers said. The size of the fish eaten by predators may play a role.
“While some predators selectively eat small fish (usually not selected by the fishery), others prey on a large range of forage fish sizes,” they said. “The degree of overlap between fisheries and predators is highly variable.”
However, work on the subject is not complete, said Hilborn. Several groups of researchers interested in the area are addressing different elements of the analysis.
“We’re doing more detailed analysis of several of the components,” he said. “A more detailed model of specific places.”
The work includes looking more closely at the interaction of key predators and some of the larger forage fisheries around the world, he said. “I expect in some of these that we’re going to find some impact – overlap between what the fishery takes and the predator takes,” he added.
Source: Fisheries Research
Title: When does fishing forage species affect their predators?
Authors: Ray Hilborn, Ricardo Amoroso, Eugenia Bogazzi, Olaf Jensen, Ana Parma, Cody Szuwalski, Carl Walters