Essington, T. E., P. E. Moriarty, H. E. Froehlich, E. E. Hodgson, L. E. Koehn, K. L. Oken, M. C. Siple, and C. C. Stawitz. 2015. Fishing amplifies forage fish population collapses. Proceedings of the National Academy of Sciences 112(21):6648–6652.DOI: 10.1073/pnas.1422020112
In 2011, humans harvested 60 billion pounds of forage fish globally (that’s more than 7 times the weight of every school bus in the United States combined!), making this group the largest fishery by weight in the world. Forage fish are small filter feeding fish like herring, sardines, and anchovies that swim in large schools. They represent an incredibly important component of marine food webs as the major link between photosynthetic algae and higher level predators like tuna, whales, seabirds, sharks and other large fish (Figure 1).
Humans harvest forage fish primarily as feed for farm animals, health supplements, and bait. The fish are typically captured by pairs of large boats that encircle a school of fish, removing large quantities of fish in one pass (Figure 2). Stocks of forage fish have collapsed often over the last several decades, putting stress on the variety of organisms that rely on these fish as an essential food source. When most species of fish experience population crashes, fishermen have to put so much effort into finding fish that it is not economically feasible to continue pursuing the fish, which provides the species with some relief. Unfortunately, the schooling behavior of forage fish often makes them valuable even when abundances are low. This makes them susceptible to being fished into very low numbers that could take a long time to recover.
Study Objectives and Methods
Forage fish populations naturally fluctuate over time. One of the primary goals of this study was to determine if forage fish stocks would have collapsed naturally even if fishing was not taking place, or if fishing is a direct contributing factor to the collapses. Naturally fluctuating populations also creates the risk of high fishing pressure occurring during times of naturally declining populations: a combination that could spell disaster for the population.
To investigate the causes of stock collapses, the authors used the RAM Legacy Database. The RAM Legacy Database consists of many years of fisheries catch data and data regarding the status of fish populations around the world. Predators are most sensitive to changes in forage fish populations when populations are small. For this reason, the researchers focused on relative changes of population sizes during times of low forage fish abundance, as well as how long stocks took to recover after a collapse, and the number of different stocks in trouble at any one time. The authors used these data to test if fishing pressure may lead to stock collapses, as well as testing a new hypothetical management rule to see if eliminating harvest once the population dips below a predetermined threshold would help prevent stock collapses. The defining characteristic of a stock collapse is that a population gets stuck at low numbers (in this case less than 25% of its average size) for an extended period of time.
First, analysis of global catch and forage fish population data found that no particular region of ocean is more susceptible to a stock collapse than any other. Likewise, no particular decade from 1950 until the current decade was more susceptible to collapse. This eliminates many environmental variables as likely drivers of stock collapses. According to the data, the best indication of an approaching stock collapse occurs when a population falls to 15-25% of its average size. This is valuable information for managers. Falling to 15-25% of average population size seems to be a tipping point that results in inevitable stock collapse. The findings also indicated that a delayed response by the fishery to alleviate fishing pressure during times of low population size was a major contributor to many stock collapses (Figure 3).
To assess if populations would have naturally collapsed even if fishing pressure was not present, the researchers calculated the minimum likely population size two years before a known collapse, then ran a of population size after this point with no fishing pressure. These models predicted that only 4 out of the 15 known stock collapses probably would have happened anyway if fishing pressure was removed two years before the collapse. This means that fishing pressure was a deciding factor in over 73% of stock collapses. The models also predicted that the minimum population size would have been six times greater if fishing pressure was removed.
Lastly, the researchers tested a new hypothetical regulation for forage fish fisheries: stop fishing completely if/when a population reaches half of its normal size. They investigated if stocks would collapse less often and recover more quickly if this rule was applied, and what the impact on the fishery would be (how much profit they would have to sacrifice from reduced catch). In this simulation, researchers saw an 80% increase in minimum population sizes and stocks collapsed less than half as often as they did without the stop fishing rule. Most notably, fishermen only had to sacrifice less than 2% of their catch to accomplish these gains.
Unfortunately, a real solution isn’t as simple as creating an on-off rule for the forage fishing industry. It is up to fisheries managers to work out how to plan for and allow some catch during times of declining populations in order to provide a stable economic environment for fishermen, while still protecting the species from a collapse (no small task!). However, these results should be encouraging to fishermen. Smart limits on fishing during a time that catches are low any way allows the population to recover quickly, rather than fishermen suffering through several years of extremely low catch after a collapse. And oh yeah, the rest of ocean life will be grateful too.
Derrick is pursuing a Ph.D. in the Organismic and Evolutionary Biology Program at the University of Massachusetts Amherst. He is interested in anadromous fish migrations, how aquatic organisms interact with their physical environment, and the impact of human development on natural systems.