Regime Shifts DataBase

Large persistent changes in ecosystem services

During the 20th century global fisheries greatly expanded in terms of effort and range. This expansion resulted in the overfishing of many fish stocks, and in some cases triggered fisheries collapses, for example the collapse of the highly productive Newfoundland cod fishery (Worm et al. 2009). While overfishing can severely deplete fish stocks, it will not necessarily trigger a fisheries collapse. A collapse occurs when once fishery pressure has been removed, stocks fail to rebuild (Hutchings 2000, Kirby et al. 2009), suggesting that the system has become locked into an alternate regime. In this case, alternate regimes are usefully conceptualized as alternative food web structures:

High abundance of a commercial fish species.

In this regime a valuable fish species is common and the fishery is highly productive. The main ecosystem services associated with high abundance of commercial fish are the production of food in first place, employment for both artisanal and industrial fishermen; followed by pest and disease regulation by healthy food webs, recreational values as recreational fishing, knowledge and educational values, as well as maintenance of biodiversity.

Low abundance of the commercial fish species.

This regime is characterized by a substantially reduced abundance of the valuable fish species. In cases where the valuable fish species performs key ecological functions, it can lead to trophic cascades that lead to an overabundance of planktivorous fish or primary producers such as seagrass, macroalgae, sponge or phytoplankton (Jackson et al. 2001). With low fish abundance, food production is the most immediate ecosystem services affected. While industrial fishermen often can move and harvest new areas or new species, artisanal fishermen often can lose their livelihoods and traditions.  

There are two key direct drivers of fisheries collapse: overfishing as a top-down disturbance, and the change in nutrients input as a bottom-up disturbance.

Overfishing reduces the population size of commercial fish species, reduces their ability to perform their functional role in the ecosystem, can destabilize their population dynamics (Anderson et al. 2008), and ultimately could change the structure of the food web. Nutrient inputs, on the other hand, affect fisheries by increasing the abundance of lower trophic levels e.g. phytoplankton and zooplanktivores such as jellyfish (Daskalov et al. 2007). Such changes in food web configuration can reduce the energy transfer to fish and hence reduce fishing productivity. These drivers can reduce the resilience of a regime making it more vulnerable to other types of regime shift, such as climate driven food web reorganization [see: Climate & Marine Food Webs]. The indirect drivers that explain overfishing and changes in nutrient input interact in synergistic ways, making their identification a challenging process (Ocean Studies Board 2006, Kirby et al. 2009).

Indirect drivers of overfishing include oversupply of fishing boats, inflexible fishing quotas, demand for fish, the development of new fishing technologies, the facilitation of trade (market connectivity), governance failures such as the failure of regulation to halt illegal fishing, and market failures such as subsidies or perverse incentives. Nutrient enrichment of marine ecosystems is also indirectly driven by runoff of fertilizer nutrients from agriculture and sewage from urban areas (Diaz and Rosenberg 2008).

A fishery collapses when a high-valued commercial species is substantially depleted and fails to immediately recover when fishing effort is removed (Hutchings 2000). It is usually associated with a substantial and persistent change in the structure of the marine community. Such a change involves changes in the species presence, their relative abundance, size and age.

When the stock of commercial species decreases or even disappears the populations of other similar species may increase. Many commercially fished species are predatory fish, such as tuna, that have a high trophic level. Removing such top predators can cause a trophic cascade to reshuffle a food web (Daskalov et al. 2007, Estes et al. 2011), especially if there are not ecologically similar species present. Such trophic cascades, can increase fish stocks of prey species (Walters and Kitchell 2001), which can in turn lead to either sequential depletion of fish stocks or the development of new fisheries (Essington et al. 2006).

The low abundance of commercial fish species can be maintained by a number of mechanisms. First, when the population of the commercial fish species falls under certain levels their population growth rate can decline from a variety of causes. This is known as depensation in fisheries, or the Allee effect more generally (Liermann and Hilborn 1997). Such an effect can occur if small schools of fish offer poorer protection from predators, smaller populations are less able to hunt or detect food, or mating success rates can decline.

Second, nutrient enrichment can favor the food web shifts towards zooplanktivores, such as jellyfish. By reducing resources for other fish, the rise of such species can suppress fisheries recovery. Fishery pressure on filter-feeding fishes like sardine exacerbates the problem by favoring jellyfish outbreaks (Jackson et al. 2001, Brotz et al. 2012, Condon et al. 2013).

Despite evidence from some well-documented cases, the degree to which fisheries declines are regime shifts remains a controversial issue for a number of reasons (Hilborn 2007, Litzow and Urban 2009). First, time series for most fisheries are only available from 1950, a time when marine ecosystems were already affected by flourishing industrial fishing, and some stocks had already been seriously depleted, especially those of large marine vertebrates like whales, manatees, crocodiles, seals, swordfish, sharks and rays (Kirby et al. 2009). This lack of long term data makes it difficult to identify causation in fisheries time series. Furthermore, marine ecological diversity and food web complexity complicates the detection of changes in fish populations due to fishing, because the signals of regime shift dynamics are small relative to ecological variation (Liermann and Hilborn 1997).

Shift from high to low abundance of commercial fish species

Since 1950 the world marine fisheries catch has expanded almost fivefold, and is currently about 80 million tonnes of fish. According to FAO, about 30% of fish stocks are overexploited, 60% are fully exploited and about 10 % are not fully exploited (FAO 2012). Global marine fish catches have plateaued and are not expected to increase. Local fisheries decline has been estimated to have affected the employment of roughly 14 million fishermen, 12 million of which correspond to artisanal fisheries (Pauly et al. 2006). There is not clear assessment of the number of stocks that have collapsed but FAO suggests about 5% of fish stocks are depleted (FAO 2012). Some collapsed stocks have remained at under 10% of their previous sizes even after decades of fishing closure (Ainley and Blight 2009). Food production, primary productivity, pest regulation, and cultural services are the ecosystem services most affected by the collapse of fisheries.  

Shift from low to high abundance of commercial fish species

An abundant fish population regime will likely better maintain marine biodiversity and will be more resilience to climate variation and climate change. Society will benefit of high abundance regime in terms of high primary productivity that translate in more food, jobs and livelihoods in small coastal communities. It will also better regulate marine hypoxia.

A review of marine fisheries in the Millennium Ecosystem Assessment offer a synthetic set of managerial options for addressing fisheries collapse (Pauly et al. 2006).

Options for enhancing resilience

There are two types of strategies for enhancing resilience. The first is to stop doing things that are eroding resilience, while the second is to being activities that build resilience. There are a number of fishing practices that could be eliminated because they damage the resilience of fisheries. For example, fishing practices that destroy habitat such as bottom trawling should be avoided. Developing alternative technologies and reduction of fishing effort are suggested. Bycatch, the killing but not consuming of unwanted fish, can also have substantial impacts and can be reduced. Indirectly, fisheries subsidies and inflexible quotas can drive fishing effort to high levels that erode the resilience of a fishery.

Options for reducing resilience of unwanted regime to encourage restoration or transformation

Direct resilience building strategies include the establishment of Marine protected areas (MPAs) (Takashina and Mougi 2014). These areas can play an important role by providing refugia for fish and increasing ecological diversity. Research suggests that they need to cover ecologically significant areas, including different habitats, ensuring connectivity among them. However there remains controversy over the extent to which MPAs shift fishing pressure to less protected sites.

Indirect resilience building efforts include efficient regulation and policing of fishing can reduce illegal fishing, while comprehensive fisheries policies at the international and national level can also build resilience if they result in fisheries policies that reinforce one another. Indirect resilience building strategies include improving the ability of fishers to exit fisheries.

Good management of fishing pressure has been able to restore fisheries stocks. For example, in the US fishing closures and implementation of management measures for bottom fishing have enabled the restoration of George Bank haddock, Atlantic scallops, George Bank yellowtail flounder, Atlantic striped bass, Atlantic Arcadian redfish, Pacific chub mackerel, and Pacific sardine. However, there is no clear strategy that works across many ecosystems or nations (Beddington et al. 2007).