A weekend for fish and the free-flowing rivers they need

@Luciano Candisani/iLCP

Mark your calendars for a weekend of days highlighting biodiversity, free-flowing rivers, and the migratory fish that depend on them.

Today, Friday, May 20, is Endangered Species Day in the US, and of course freshwater species are highly threatened. According to the IUCN Red List of Threatened Species, about 27% of assessed freshwater species are threatened with extinction; almost 30% of assessed freshwater fishes are threatened.

Sunday, May 22, celebrates the International Day for Biodiversity. Despite the scale and severity of threats to freshwater biodiversity, as documented in the 2020 Living Planet Report, there has been concern that freshwater ecosystems and their biodiversity have not been appropriately addressed in the Convention on Biological Diversity’s (CBD) Post 2020 Global Biodiversity Framework. The latter point was recently highlighted by an open letter to the CBD, signed by over 2000 scientists, conservationists and institutions from 94 countries.

Sandwiched between those two days is another very important one: May 21 is World Fish Migration Day (WFMD). There are 450 events in 75 countries celebrating the importance of migratory fishes and the free-flowing rivers on which they depend.

The threats to free-flowing rivers resulting from river fragmentation caused by dams and other water infrastructure, with associated habitat modification and changes in water quality, have been well documented. Dams act as barriers to migratory fishes moving longitudinally along the river, and the changes in water and sediment flows can change flood and nutrient cycles that are necessary to trigger fish migrations for species that migrate laterally out onto flood plains. Water released from reservoirs behind dams tends to be cooler, and lower in sediment than the natural condition of the river, creating stressful conditions for native fishes while often promoting the colonization by invasive species such as trout.

The Living Planet Index for Migratory Freshwater Fish, published in 2020, showed that, globally, migratory freshwater fish have declined by an average of 76%. Many of these species are well known, anadromous species, that migrate from marine waters to rivers to spawn such as various species of salmon. These species are the basis of important commercial, artisanal, recreational and indigenous fisheries. Many commercial fisheries are more heavily managed with support from aquaculture, but artisanal and indigenous fisheries tend to rely on wild stocks that migrate upriver. For example, in the western USA the Snake River Basin was the major producer of salmon and steelhead, critical for the livelihoods of indigenous communities. However a Nimiipuu (Nez Perce) study published in 2021 showed that wild populations of Chinook salmon (Oncorhynchus tshawytscha) were declining 19 percent every year, with dams being part of the problem. Species like the ayu (Plecoglossus altivelis) spawn in rivers, the fry migrate downstream to the sea for a short while before migrating back upstream. They are both commercially and important for food, and culturally important (see From Sea to Source 2.0, p.82–83). Eels, on the other hand, move from freshwater to the sea to spawn; but they are also a migratory species important for fisheries, and threatened by river fragmentation.

However, ‘potamodromous’ fishes — those that migrate within freshwater bodies, either along the river, or out onto flood plains during periods of floods, or between rivers and lakes — are even more threatened than ‘diadromous’ fishes, migrating between fresh water and salt water. According to the The Living Planet Index for Migratory Freshwater Fish, potamodromous fishes have declined by 83% on average, compared to a 73% decline for diadromous fishes. Many of these potamodromous fishes are also very important for inland fisheries. The Amazon basin has a large number of migratory fishes, and these migratory species are among the most important fishes captured in the commercial fisheries. Yet they are also highly threatened. For example, the yatorana (Brycon amazonicus) migrates along rivers and into floodplains, and is the most important target species in the Amazon for recreational and commercial fisheries (see From Sea to Source 2.0, p.88–89). But it, and thus the fisheries for it, are threatened by the development of dams that disrupt its migrations.

Large dams, usually for hydropower, are the most commonly discussed mechanism of river fragmentation that can block fish migrations and threaten fisheries. And, with calls for a doubling of global hydropower capacity by 2050, these threats are likely to get worse. However, smaller physical barriers such as sluices, weirs and culverts that may be associated with water control for agriculture are also significant, and in some cases greater, threats. World Fish Migration Day regularly includes events focused around Lake Tana, Ethiopia (including three events this year) and its species flock of Labeobarbus, several of which are endemic to the lake. These species migrate from the lake into its tributaries to spawn (see From Sea to Source 2.0, p.66–67). The species of Labeobarbus are an important fishery resource for people living around the lake, supporting over 6000 fishers. However, the fisheries have significantly declined due to various threats, including the development of dams and weirs. Similarly, a study in Myanmar highlighted the importance of considering the needs of migratory fishes when developing these types of agricultural infrastructure. A decline in inland fisheries in Myanmar could be associated with the development of sluices, weirs etc., and that can also decrease the capacity for sustainable development and to meet Sustainable Development Goals. A recently published study has also highlighted the fact that inland fisheries are insufficiently addressed in the Sustainable Development Goals.

Disturbance of multiple fisheries along a river will have compound effects due to social and ecological interactions along the river. For example, a study of the 3500 km long, free-flowing Paraguay-Paraná river corridor highlights that it supports several small-scale fisheries dispersed along the length of the river, and these represent a complex mosaic of socio-ecological systems requiring appropriate governance. Hence, any disturbance of these fisheries will have complex effects due to multiple and varied social-ecological interactions.

Many migratory species are important for recreational fisheries — for example, species of tigerfishes in Africa; or salmonid fishes in many parts of the world, including the large, charismatic species of taimen in east Asia. These are prize fishes drawing high-paying recreational fishers. The Bita River in Colombia, which is protected as a Ramsar site to maintain its status as a free-flowing river, attracts more than 1000 sport fishermen during the low water period (December to March), bringing in an important source of revenue. The recreational fishery interest in some of these species has stimulated conservation programs focused on engaging anglers as stewards in the process. For example, the Mahseer Trust in South Asia, or the Yellowfish Working Group in South Africa.

Attention on migratory fishes and their fisheries often focuses on the large, charismatic species. Unsurprisingly that is especially so for recreational fisheries — since these are focused on ‘trophy fishes’. However, there are also many small species of migratory fishes that are important fishery resources. Trey riel — small mud carps of the genus Henicorhynchus provide extremely important fishery resources to millions of people in the Mekong river. They migrate out onto flood plain areas, and when they return to the river during drawdown, thousands of tonnes are caught and processed into fish paste which can be stored and used to enrich the food throughout the year. Therefore, any change in the seasonal flows caused by upstream dams represent a threat to these fishes, the associated fisheries, food security and nutrition.

The timing of World Fish Migration Day this weekend is most appropriate. Coming as it does the day before the International Day for Biological Diversity, it can serve as a reminder of the importance of ensuring the Post 2020 Global Biodiversity Framework adequately considers the issues of properly conserving and managing river connectivity, freshwater fishes, and the fisheries they support, within its relevant targets.




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