Clever Measures Track Fishing Pressures

A Honduran small-scale fisherman on a boat setting out to fish.
A fisher from a Honduran small-scale fishery sets out on a fishing excursion. (International League of Conservation Photographers/Luciano Candisani)

Imagine taking a few measurements from a freshly caught fish and from that small bit of information being able to tell precisely where in the ocean it came from. Fishing is vital in many parts of the world, including the Caribbean where communities in countries like Honduras rely heavily on seafood for income and food. But commercial fisheries, recreational fishers, and tourists have harvested from the waters surrounding the region’s islands for centuries, and many fish populations in the region have decreased dramatically. 

Typically, to maintain sustainable fisheries, the Caribbean Fishery Management Council implements management strategies for commercial fish populations; such as marine protected areas (MPAs), no-take zones (NTZs), and territorial user rights fisheries (TURFs). All of these zones are designated areas of the ocean where there are limits on the amount and types of catch allowed. The regulations are designed to protect particular areas of Caribbean waters in order for fish populations to grow and reproduce undisturbed. But enforcing these types of fishing regulations can be difficult. The sheer size of the ocean makes patrolling it a challenging task. 

One way to potentially improve enforcement is a check at port as boats come in with their catch. The missing piece was a reliable test to determine where the fish are from. Steven Canty from the Smithsonian Marine Station at Ft. Pierce, FL used measurements of the external shape and structure of individual fish as a method to identify their specific habitat range. Canty’s study shows this simple, cost-effective technique could prove to be a reliable way to manage small-scale commercial fish populations throughout the Caribbean and, potentially other areas of the world.

Canty focused on the yellowtail snapper in a Honduran community fishery and compared three different methods of determining a fish’s origin to see which was the most dependable and cost-effective: genetic analysis of fish tissue, elemental analysis of fish otoliths (or ear bones), and morphometrics. This last is an analysis of lengths between obvious fish body parts, such as the base of the dorsal fin or tail fin, and its full body length. Body shape measurements were almost 80 percent accurate in determining a fish’s origin, while otolith and genetic analysis were 54 and 52.4 percent accurate respectively. Fish measurement was also the cheapest and fastest method of the three as it could be done immediately at the dock without specialized equipment. 

A yellowtail snapper lays on a blue cutting board where it is measured.
The pins in this image show the 10 reference points that can be used to characterize the body shape of an individual fish. The differences between the body shapes were subtle across individuals; however, they were sufficient to determine where a fish was caught with an accuracy of approximately 80 percent. (Steven Canty, Smithsonian)

In measurement analysis a scientist or fisheries manager uses set anatomical points on a fish to determine the fish’s unique shape. Fish that live in different habitats, even if from the same species, will have slightly different forms. Depending on which shape the fish has, a knowledgeable manager could know after just a few measurements where the fish came from. 

This method does not work for all fish species. It works best with those that stay within a relatively small home range—in this study the individual yellowtail snapper populations stayed within a range of 2 square kilometers or less. Highly mobile fish species with large ranges, like tuna or swordfish, would not have distinct body shapes linked to a particular area of ocean. Fish anatomy analysis also does not work well with open ocean fish populations that live in very similar environments. 

Reef species with a fairly small home range (or territory) are the best candidates for this type of analysis—highly valuable commercial fish species like grouper, snapper, and grunts. Canty estimates that up to a quarter of global commercial fish species fit the requirements for body analysis to work well in identifying origin. 

It is unclear exactly what environmental cues in the habitats cause the different traits. In the yellowtail snapper study three locations were surveyed that were anywhere from 5 to 60 km apart. Each had different prevailing currents, depths, topography, and water chemistry. Any of these factors could be causing the unique shapes of their resident yellowtail snapper population. Canty suspects that differences in diet could also be a key factor in the slight differences. 

To further improve the accuracy of the unique signature for the three habitats studied, more snapper measurements need to be analyzed. Whatever the environmental causes, this method could become a standard tool used for other fish species and environments to help maintain important fish populations for years to come. 

August 2018