Defining sustainability
There is now a multitude of meanings of sustainability and how to define “sustainable” seafood. A common theme to all discussions on sustainability is living within limits and the capacity of natural ecosystems to indefinitely produce the goods and services we want. For example, Murawski (2000) discusses how ecosystem-level sustainability implies “systems are managed for the highest net benefits to society consistent with other biological objectives.” Any population, or system, that is harvested to maintain forever maximum sustainable yield or near it would meet many definitions of sustainability. If future generations wanted to return such a population to an unfished state, they would have the option of ceasing all harvest, and in theory, the population would return to its preharvest condition. Now consider a fishery where the annual exploitation rate is higher than would produce long-term maximum yield so that the population fluctuates at a lower level than would produce maximum yield. If such a fishery can be sustained indefinitely, then it too would seem to meet the Brundtland definition; future generations could choose to harvest at a lower rate and the population would increase. This could be called “sustainable overexploitation”.
Major differences in perspectives on sustainability revolve around the extent to which the use of a resource modifies other components of the ecosystem. In a fishery that is harvested to produce long-term maximum sustainable yield, fish abundance will be lower than it would be if not harvested at all. However, add to this scenario the unintentional catch of another species, commonly called bycatch. If the level of unintentional catch is low enough, the impact of this fishery on the bycatch species may simply be to reduce the mean abundance of the nontarget species, but if the unintentional catch is too high and the bycatch species has a low reproductive rate, that species may go locally or even globally extinct. In this case the targeted fish species is sustainable, but we will have eliminated options to take benefits from the bycatch species for future generations.
Much of the controversy over sustainability appears not to be centered on the potential for long-term yield of the resources but on how much alteration to the ecosystem we are willing to accept. Fishing undoubtedly changes the trophic structure of an ecosystem, and fishing one species may make other species more or less abundant even if not threatening local or global extinction.
Groups concerned with the status of seabirds, for instance, may consider fisheries that reduce bird food availability beyond some point to be. In view of this, how do we measure the environmental sustainability of marine ecosystems?
Single-species population dynamic
The theory of exploited populations suggests that the mean abundance of the population will decline as the exploitation rate increases and that the long-term mean yield will be maximized at an intermediate exploitation rate. Managers seeking to maintain long-term MSY search for the exploitation rate with the highest long-term harvest, but in theory almost any exploitation rate that does not lead to extinction of the population or cause a flip to an alternative enduring state is sustainable (in the sense that they can be maintained indefinitely). Flipping into a permanent alternative state would deprive future generations of the potential benefits from the species. In many places, exploitation rates above the level that would produce MSY are called “overfishing”.
As fishing mortality is increased, sustainable yield initially increases, then beyond some point it declines. This simple relationship is derived for a logistic growth model, but can be shown to result from a wide range of life histories and population dynamics. For instance, age-structured or size structured models provide a similar relationship, the abundance declines with exploitation rate, and yield is maximized at an intermediate value. It is an exceedingly reassuring view of how populations behave, because the exploitation rate can be reduced at any time and the population will rebuild to its higher levels and can, in theory, rebuild to its unexploited state if harvesting is stopped.
However, there are many ecological relationships that can provide different perspectives. The ones of most concern are thresholds or tipping points (Kelly et al. 2015) in either population size or exploitation rate that lead to irreversible changes. Perhaps of the greatest concern are mechanisms known as depensation that can lead to a threshold population size below which the population might never recover. Concern about possible low abundance thresholds and the long recovery times to rebuild stocks from low abundance has caused management agencies to attempt to avoid low levels of abundance. Large-scale meta-analysis suggest that there is little evidence for depensation, although it certainly cannot be ruled out in individual cases. Thus, while the weight of the evidence is that stocks depleted to low abundance will generally recover if fishing pressure can be sufficiently reduced, provided the environment has not changed, almost all past considerations of fisheries sustainability have suggested that there are lower limits on abundance below which stocks are not considered sustainable.
Fishing exerts selective pressures on stocks, and one of the most ubiquitous and striking examples of life history responses to fishing is the lowering of the age and size at maturity of heavily exploited stocks. Fishing increases the total mortality rate, individuals are less likely to live to older ages, and individuals who delay reproduction until older ages are unlikely to survive to reproduce. This is almost certainly an evolutionary response to fishing pressure. The impact of such changes is twofold: the long-term yield available at any exploitation rate will be lower than the simple logistic theory, and the higher exploitation rates otherwise considered sustainable could lead to extinction of the population.
There is a considerable literature documenting major changes in fish stock abundance unrelated to fishing, and recent meta-analysis suggests that irregular and often abrupt changes frequently occur in the key parameters (recruitment, somatic growth, and natural mortality) either from natural or anthropogenic causes. These abrupt changes are often called regime shifts, and such shifts can alter aspects of ecosystem dynamics and the sustainability of exploitation. For instance, if a stock shifts into a less productive regime because of changing ocean temperatures or a decreased food supply owing to fishing, the sustainable yield and exploitation rate that would maximize longterm yield may both decline. While this does not mean the stock is no longer “sustainable”, it does mean that sustainable management in an unproductive regime will need to be different than management in a productive regime. If a population shifts into a more productive regime, previously unsustainable exploitation rates may become sustainable.
In summary, all the single-species evidence available suggests that species can be sustained across a range of fishing pressure and that stocks will rebuild when fishing pressure is reduced, unless there have been externally induced changes in the environment or a tipping point has been crossed. Stocks may be sustainably overfished in that they can sustain exploitation rates in excess of those that would produce MSY and recover to MSY levels (and beyond) if fishing pressure is reduced, but whether such overexploitation is desirable is a societal matter.
However, few fisheries catch a single target species; many fisheries capture a broad mix of species.
Multiple species caught in the same fishing gear
The theory of mixed stock fishing can be divided into two parts: the technical interaction due to the fact that multiple species are caught with the same fishing effort and trophic interaction that considers the predator–prey and competitive dynamics of ecosystems.
The theory of management and maximization of sustainable yield from mixed-stock fisheries has received ongoing consideration. The problem occurs when two (or more) stocks being jointly harvested have different optimum exploitation rates. To illustrate with a simplified example, we assume we have a productive stock where MSY would be achieved at a harvest rate of 30% per year and an unproductive stock where MSY would be achieved at 10% per year. If both stocks have the same potential yield, then long-term MSY is achieved by applying a harvest rate slightly higher than would maximize the unproductive stock so it would be slightly overexploited. However, if the potential yield of the unproductive stock is small in comparison with the productive stock, then long-term yield will be maximized by overfishing the unproductive stock, and, depending on the relative optimum exploitation rates and abundance, the long-term yield may be maximized by fishing so hard that the unproductive stock goes to local extinction.
This is a problem for nearly all mixed-stock fisheries, since there are almost always some unproductive and productive stocks in the mix of what is caught. Trying to capture the potential of the productive stock while protecting the unproductive stock has been an ongoing management concern in a wide range of fisheries, both within and between species.
The response of some fisheries managers has been that all stocks that are assessed must be fished at rates less than that which would produce maximum sustainable yield (FMSY) resulting in substantial lost yield overall. While some would hold such lost food production acceptable, those concerned with food security may not find it so. Consequently, a number of solutions that try to meet both expectations by reducing exploitation rates on unproductive stocks while allowing harvest of productive stocks have been proposed and implemented. Core habitat for the least productive stocks can be closed to provide a refuge. Gear modifications can be found that reduce the relative effectiveness of gear on the least productive stocks. Finally, individual incentives have been provided to fishermen to find time, places, and fishing methods that minimize catch of unproductive stocks, such as individual vessel quotas on both unproductive and productive stocks. Using the concepts from single-species management discussed earlier, any fishing policy that may overfish unproductive stocks but allows them to recover in the future would meet the Brundtland definition of sustainability.
None of this, however, takes into account resilience to external perturbations such as climate change, and there is growing evidence that the long-term productivity of a mix of stocks depends on maintaining the portfolio of stocks over time, so that while short-term yield may be maximized by severely depleting unproductive stocks, stocks that are unproductive during one regime may be the productive stocks of the next environmental regime.
Ecosystem dynamics
Model results Fish stocks do not exist in isolation, affected only by removals from fishing; species in a marine ecosystem interact through predation and competition. Thus, if we fish a single species, the abundance of prey of that species would be expected to increase, whereas the predators of the target species might decline because their food supply is reduced. These interactions are assessed in the ecosystem approach to fisheries (EAF) or ecosystem-based fisheries management (EBFM).
A range of ecosystem models that consider these trophic relationships have been used to evaluate ecosystem-wide impacts of fishing. While the implementation of these ecosystem models differs in many ways, a common result is that the ecosystem-wide yield behaves much like a single-species model. As exploitation rate increases from zero to higher levels, long-term ecosystem yield increases, eventually reaching a maximum, and then as exploitation rates increase further, the total yield declines.
A modeling study by Garcia et al. (2012) found that when fishing broadly across an ecosystem (exploiting all non-microfauna, including jellyfish, macroalgae, small-bodied pelagics such as krill, finfish, and even high-trophic-level species like marine mammals), not only was the sustainable catch of this entire assemblage of species much higher (1.5–2 times greater than for selective fisheries typical of North American, Australian, and western European nations), but there was little if any decline in that yield until the exploitation rate was very high. The entire shape of the curve was (typically) more skewed to the right than seen when focusing on traditionally targeted finfish; this did mean, however, that when exploitation rates rose very high, the declines in catch were precipitous. There was always a biodiversity cost of high exploitation rates, with some of the traditionally preferred species disappearing and being replaced by other species that can sustain very high exploitation rates. However, the rate of loss and replacement was not so rapid when fishing broadly across the ecosystem than when selectively targeting traditional finfish. This is because the application of pressure across much of the system was not as destructive for system structure and connectivity as the selective removal of specific nodes.
Trophic models show the same basic trade-off that is found in mixed, single-species models; if you want to maximize the yield from a mix of stocks, the less productive stocks will be overexploited. Moreover the tension between objectives for different components of the ecosystem will remain (e.g., between targeting small pelagic fish and allowing for consumption by species of conservation concern).
Consideration of alternative harvesting regimes (e.g., balanced harvesting) is a growing area of research because there is a need for open discussion of what different patterns of fishing mean across objectives — e.g., intentionally shifting to targeting smallerbodied and more productive forage fish— and whether that is considered desirable by society. Even if concepts like balanced harvesting are found to be sound and desirable in theory, the practicalities involved are quite challenging, not the least of which is re-educating the palate (and markets) of more selective cultures and addressing the economic considerations of fishing fleets. Not all harvested biomass is equally sustaining (or attractive) to all people, further highlighting the social and economic aspects of sustainability.
