Target species and the ecosystems in which they live are inextricably tied, and fishing affects ecosystems in ways beyond just the removal of the targeted species. These indirect effects, along with other anthropogenic effects like coastal development and pollution, may cumulatively have a greater impact than direct fishing pressure alone for some species. The effect of fishing on the dynamics and health of marine food webs is an important consideration in EBFM.
The effect of fishing on food webs rarely grabs headlines in the media. However, as these impacts are better understood, the issue is expected to be raised more publicly by scientists and activist environmental non-governmental organizations (NGOs). The challenge up to this point has been a lack of data and clear documentation. Negative effects of fishing on food web dynamics may occur over long time frames. These effects are not as apparent as other fishing impacts to the ecosystem, like trawling through a sensitive coral habitat or catching dolphin in a net while targeting tuna. However, acute impacts, such as intense fishing in a confined area in a short time frame, can also cause disruption to marine food webs. Regardless of the type of effect, this paper defines adverse fishing impacts to food webs as those that alter ecosystem resilience and function in a negative way.
Ecosystem-wide food web impacts can begin at any point in the food web. Removal of top predators can destabilize the food web by allowing for the explosion of their prey populations, which can then cascade down through the food web. Removal of prey species at the bottom of the food web can deprive predators of food, having negative ramifications for their populations. But even the removal of competitors from within the middle of the food web can have a ripple effect as their niches are filled by other species. Regardless of which situation may be occurring, scientists recommend understanding the food web from which fish are being removed and setting management measures to maintain ecosystem function and health.
Complex models to simulate ecosystem and food web dynamics are being developed all over the world. While they can be very informative and a useful tool for managers when making decisions, they require significant data inputs. Information on abundance, distribution, and diet is needed across multiple species. Some data and tools are available to understand the impacts and to minimize adverse impacts, but more is needed. Yet, managers can take action to consider fishing impacts on marine food webs with information and tools available today. Simple ecological risk assessment methods for data-deficient fisheries have been developed and are increasingly used to guide EBFM. In addition, precautionary management measures can be put in place to enable some harvest while longerterm data collection and monitoring programs are implemented.
Conventional fisheries management follows a single-species approach with a total allowable catch (TAC) set annually in order to maximize the harvest of the target species without depleting the fish population. Generally, fishing impacts to marine food webs are not clearly factored into conventional harvest scenarios, but this is changing. In this report, Sustainable Fisheries Partnership (SFP) provides the following case studies where fishing effects on the marine food web are or should be considered when managing the fisheries.
• The Antarctic krill fishery is managed with precautionary limits that enable the harvest of krill while preserving its role as a key prey species for many other species in the ecosystem. A model that features krill at the center of a food web is being developed.
• Heavy fishing on cod and other high trophic level species in the northwest Atlantic have led to a drastic restructuring of the entire food web. After such a drastic restructuring of the food web it can be difficult, if not impossible, to restore ecosystems to their original state.
• Models have been constructed that describe pelagic food webs in the Eastern Pacific Ocean. Changes in food web dynamics play an important role in fluctuations of tuna population size, and must be considered as management regulations are created.
• A number of ecological indicators have been identified as key factors in the status of wild Pacific salmon populations. These indicators are gradually being integrated into management in a move towards EBFM.
• Continued harvest of Barents Sea capelin combined with a climate-driven downturn in the population size may have contributed to the collapse of the fishery in the late 1980s. Capelin are the primary food source for cod, a major fishery in the Barents Sea. Fishery managers now have a control rule in place requiring a moratorium on harvest when the population shrinks below a minimum threshold. This control rule ensures that there is enough capelin to produce future generations (allowing the fishery to re-open in a few years) and provide food for Barents Sea juvenile cod.
A number of best management practices are drawn from the literature and case studies. To sustain marine food webs, fisheries management uses (1) depictions of trophic links and biomass flows for food webs, (2) a management framework with environmental targets for food webs and ecological indicators for measuring progress towards those targets, and (3) alignment of harvest scenarios and catch levels to meet those environmental targets.
Seafood buyers can support best practices in fisheries to conserve food webs with the following practical actions:
1. Prioritize source fisheries that have the most urgent need to address fishing impacts on food
webs as part of EBFM.
2. Where a significant negative impact of fishing on a food web is suspected and EBFM is not being applied, encourage fishery regulators to consider precautionary management measures.
3. Encourage fishery regulators to identify ecological indicators and set environmental targets where they do not already exist.
SFP is available to assist companies and provide guidance for taking these actions.
June 2014
