Have you ever wondered how far the fish you see when diving in the kelp forest move, or if you see the same fish twice when you come back to your favorite dive spot? Since there are no clear boundaries in the ocean, fish are often thought of as moving freely over large areas. Some fish such as the large pelagic (open ocean) species swim across the entire ocean on their migrations. But what about the species we see on the rocky reefs along California’s coast?
This is not only an interesting question because it will tell us how likely it is to see the same fish twice, but it also has important consequences for the management of fisheries and the conservation of these fish species. For example, movement of individuals in and out of protected areas determines how much protection a marine protected area (MPA) of a given size will provide to fish within its boundaries. Movement of individuals will also determine how quickly an area that has been heavily fished will be replenished by individuals moving in from other reefs.
In a recently published synthesis of what we know about the movement of reef fishes along the temperate west coast of North America, Dr. Jan Freiwald, Director of Reef Check California summarized movement studies done since 1950 and on reef fishes along the coast (Freiwald, 2012). He found that the large majority of species studied did not move very far. In fact, for 80% of the species analyzed the maximum movement distance was less than 1.5 kilometers and many of them moved on scales of only a few hundred meters. Further, in all species that have been studied the movement distances of most individuals are much less. Another interesting finding of this study is that the reef fish along the west coast swim within a very defined area, called a home range. What this means is that they move back and forth within a region of the reef rather than moving along the coast in a linear fashion - never returning. We are used to this kind of movement from other animals – they stay within their range or territory but fish have often been thought of as moving without boundaries.
Such limited movement suggests that the neighborhoods in which individuals interact with each other or their ecological community (ecological neighborhoods) are rather small and their interactions are limited to local reefs. The finding of limited movement in temperate reef fish species will have important consequences for understanding the ecology of these species and also for understanding and modeling population connectivity in a management context. Small movement ranges, for example, suggest that MPAs even of relatively small sizes will be effective in protecting individuals and will give them a chance to grow larger within their boundaries. For divers, these findings mean that it is likely that you will see the same individual fish in their home ranges as you visit your favorite dive spots.
The study can be found at: Freiwald, J. (2012). Movement of adult temperate reef fishes off the west coast of North America. Canadian Journal of Fisheries and Aquatic Sciences 69(8): 1362-1374.
|Example of small home ranges of temperate reef fish. The colored lines outline the home ranges of several individuals of kelp greenling on a map of the seafloor (gray shades indicate reef relief). These small areas are on the order of a few hundred square meters in which individuals remained for several years. As higher resolution movement studies become available, we are finding that many species exhibit small home ranges such as these.|