Understanding the Mosquitofish Population in the Groot Estuary
Sometimes the most exciting results are not what you find, but what you don’t. In June, we visited the Groot Estuary to survey the invasive western mosquitofish (Gambusia affinis). Unlike most fish surveys, where the aim is to catch as many fish as possible, we were hoping for exactly the opposite. As we worked our way through the estuary, every empty sweep of the net became a little more encouraging. By the end of the survey, we had begun to hope that the fish we didn’t find might prove just as important as the ones we did.
Mosquitofish are small freshwater fish, growing to only about 4 cm in length. They tolerate a wide range of environmental conditions, including changes in salinity, allowing them to survive in both freshwater and brackish environments. Mosquitofish thrive in shallow, muddy and vegetated habitats. They also reproduce rapidly, allowing populations to establish and spread quickly. In freshwater ecosystems, mosquitofish have been shown to reduce populations of native species through a combination of competition, aggression and predation. They are highly aggressive towards other fish and are opportunistic feeders, consuming everything from algae and aquatic insects to crustaceans, amphibian larvae and even small fish. Combined with their high reproductive capacity, these characteristics have earned mosquitofish their reputation as one of the world’s most successful invaders.
It was therefore a concern when mosquitofish were first recorded in the Groot Estuary during fish community surveys in 2010. Their discovery prompted monthly surveys of 15 sites along the estuary between 2011 and 2013 to determine how widespread they had become. The results showed that the species had established itself throughout much of the estuary and was particularly abundant in shallow, muddy areas with aquatic vegetation. By that stage, the population was already well established, and attempting to remove the fish mechanically would have been both extremely difficult and unlikely to succeed.
Recently, the Groot Estuary experienced major flood events that dramatically altered the system. Although mosquitofish are renowned for their ability to tolerate a wide range of environmental conditions, they are less tolerant of rapid environmental change. Considering their preference for slow-moving, sheltered habitats, the rapid environmental changes associated with flooding may therefore have affected the population, with the possibility that many individuals were washed out of the estuary.
Sampling the same sites that had previously been surveyed between 2011 and 2013, we found relatively few individuals—just ten mosquitofish. Seven males were collected from one site and three from another, with the two sites located some distance apart. We found no females. While not finding females does not necessarily mean they were absent from the estuary, these results suggest a dramatic decline compared with the high abundances previously observed. Whether the recent floods are responsible for the apparent decline in mosquitofish remains uncertain.
Interestingly, studies from the nearby Wilderness Lakes System have shown that mosquitofish populations naturally undergo boom-and-bust cycles, with winter mortality rates reported to exceed 80%. When populations become smaller, females compensate for these declines by investing more energy into reproduction, allowing numbers to recover rapidly once environmental conditions improve. This means that even a small surviving population could quickly re-establish itself.
Nevertheless, finding very few fish felt like an unexpected window of opportunity to monitor and manage this invasive species before it once again became abundant. Whether those empty nets represent a genuine opportunity to control mosquitofish in the Groot Estuary or simply a temporary lull before the population re-establishes itself is something we do not yet know. It may now be worthwhile to re-establish monthly monitoring of the Groot Estuary, not only to determine whether the mosquitofish population recovers, but also to remove any mosquitofish encountered during the surveys before their numbers increase again.
