Fishing, a window into marine ecology

Many of us look at nature, particularly at a healthy ecosystem, and see chaotic color, a messy morass, a lawless lot of life forms. The science of ecology shows us otherwise. We find collections of animals and plants within specific ecosystems, which themselves are subdivided into habitats with distinctive characteristics.

One of the things I like about fishing is that it’s a chance to witness the local ecology. I started fishing as a little kid, mostly with my maternal grandfather. Ben Harris was a third-generation San Franciscan, born in 1898 and witness to massive environmental and economic change in the areas surrounding his and my hometown. He used to take me fishing for sunfish in man-made lakes, which was mostly what was left of fishing opportunities in the area by my lifetime. In high school, I had a serious love interest whose parents owned land in the Trinity Alps of northern California. She, her dad (named Sherwood, who better to explore a forest with?), and I used to go fly fishing for trout on the stream that flowed through that small plot of land. This was a vastly different experience than catching sunfish in a man-made lake: the fish were big enough to eat (and tasty at that) and the fishing technique required hiking and casting skills. The biggest difference, though, was the omnipresence of ecology in our fishing strategy. We chose flies to match naturally occurring insects in that place and season so that they would be recognizable by the trout as common food. We identified likely trout pools based on their suitability to shelter and feed a big fish. We aimed our casts upstream from these pools, choosing a placement that would allow the fly to drift naturally into them. To catch our trout, we had to be in tune with their ecology.

Bolivar Cay in the Seaflower Biosphere Reserve, Colombia

As one of my favorite research projects, my colleagues and I advised managers based on similar ecological patterns. In 2000, I had the great luck to lead two expeditions to the future Seaflower Biosphere Reserve in the Caribbean waters of Colombia. The local government agency wanted advice on how to design the Reserve, including no-fishing and other restricted access zones, and relied on me because of my expertise in marine protected area design. They already had some sophisticated maps of coral reef distributions, but wanted to consider fish communities for the sake of healthy fisheries and vibrant diving sites. Our expeditions consisted of collecting information on benthic communities, including coral, sponges, algae, and other bottom dwellers; and fishing assemblages. We then used statistics to categorize sites, grouping ones with similar species assemblages and separating those with differences.

Not surprisingly, we found that the benthic communities fell into categories that varied depending on depth, proximity to land, and whether the reef was on the exposed or protected side of the island. These categories matched what my team had expected prior to the surveys, based on our understanding of Caribbean coral reef ecology. What was more surprising were the fish assemblages. Despite the mobility of fish and the potentially disruptive effects of fishing, the fish assemblages grouped exactly the way the benthic communities had. You can read more about our work here. Our work confirmed the ecological basis of our definitions of habitat types, and suggested that the Colombian government could protect a representative portion of their coral reef ecosystems if they included similar percentages of each habitat type in their protected areas. We were also able to reassure the government that sites of the same habitat type would have similar value, allowing them to work flexibly with the local fishing community to devise the most acceptable version of fishery closures.

Dolphinfish, Coryphaena hippurus

These patterns can also be viewed through fishing activity itself. I worked with a graduate student, Kristin Kleisner, on a project to look at the ecology of pelagic fishes—those that live up in the water column. We examined oceanographic conditions using satellite data and fish abundance using catches as recorded by observers, who took down details of fishing strategies and catches (including discards). With these two data sets, we found that dolphinfish (Coryphaena hippurus) showed distinct patterns. Inshore, they were more likely to be caught in shallower water and near fronts—places where warm and cold water come into contact. Offshore, they were more likely to be caught in deep water and farther from fronts. Once again, ecology shined through in patterns of fish catches, and such patterns are integral to the strategies of fishing captains who target pelagic fishes.

These patterns play an important role in the management of fisheries through the challenge of estimating fish abundance. Ideally we would have regular, independent, scientific surveys for this purpose that sampled a wide variety of conditions in a controlled fashion. However, most often we rely on catch per unit effort (CPUE) from the fishing industry. If a fish stock is abundant, we can assume a fishing operation will catch more fish per unit effort (e.g., hook-hour, trap-day). We regularly assume that CPUE is a good, linear indicator of abundance. In other words, if CPUE doubles, we assume the fish stock has become twice as abundant. If it drops to half its original value, we assume the same of the stock abundance. We have major challenges with using CPUE as an indicator of fish abundance. Fishing activity is not random or consistent. In fact, we expect smart fishermen and women to adapt their strategies to maintain high CPUE, even if fish abundance drops. In order to see through these changing strategies, we need information about fishing behavior so we can account for changes, a process known as standardization by tracking different fishing strategies and looking for changing CPUE within each strategy. Often we lack sufficient information to do so. This problem becomes even more complex in fisheries that target multiple species. In these fisheries, which are the norm rather than the exception, changes in effort can also involve a shift in the target species. When this happens, we may see dramatic drops in the CPUE for one species simply because another becomes a more profitable target. If we have a better understanding of the underlying ecology and how it gets represented in catches, we can do a better job of standardizing effort and understanding and accounting for targeting.

Recently, I tried out a new idea for handling fishing effort. When I was in Colombia a month ago, we examined data from many fishing trips that all used hooks on fishing line. Some of these were used at the surface, others in midwater, and still others deep down. Trips were categorized as surface, midwater, deep, or some combination thereof. We had a hard time standardizing effort based on these reports because most trips were a mix of strategies without any indication of which were the main emphasis. We did have records of fish caught on the trip, and I thought it might be interesting to see if we could see some ecology reflected in the data. Even with messy data from fishing trips that ranged across a large area and varied in time from hours to days, the ecology did shine through. Certain trips tended to catch deep-dwelling species while others tended to catch certain species of shallower water fish. We are now working on an index of how heavily each trip should be weighed when calculating CPUE, with heavier weights for trips that caught a collection of species that were ecologically similar to the one of interest. Fishing does indeed offer a window into marine ecology, and we can do a better job of understanding fisheries by paying attention to the view in that window.

All my best,

Josh