The first thing most people tend to notice about a shark is its dorsal fin. Can’t blame them: it’s the poster-child for slicing through the water and inspiring everything from awe to anxiety depending who you are. Thanks, Jaws. But sharks have more than just a dorsal fin and in Puerto Rico, it is another fin entirely that is helping scientists better understand one of the Caribbean’s most complex conservation challenges.
The fin in question? The anal fin. It’s a small fin located on the underside of many shark species. It is not particularly impressive to look at, unlike the dorsal or caudal (tail) fin, and it has little commercial value. Yet according to new research, this overlooked body part may hold the key to monitoring shark fisheries more effectively across the Caribbean. “I can’t really take credit for the idea [of using the anal fin,]” says lead author Dr. Devanshi Kasana who performed this work while an independent consultant to Mote Marine Laboratory & Aquarium and is currently affiliated with Oceana. “The method was actually developed and tested first in Belize, where researchers found that shark anal fins can be surprisingly informative [by allowing us] to identify species and estimate the size of the shark that was landed. We sometimes describe them as a kind of ‘biological logbook’ because each fin contains information about what was caught.”
Puerto Rico is home to a small-scale shark fishery driven largely by local demand for affordable shark meat. Compared to industrial fisheries, these operations are relatively modest with fishers often working close to shore and selling their catch through local fish markets. But like many small-scale fisheries around the world, managing them can be difficult. One major challenge is knowing exactly which shark species are being caught. That might sound like a simple question — after all, if a shark is landed, surely someone can identify it? But the reality is that shark fisheries often involve multiple species that can look remarkably similar, especially after processing, so it can be hard to accurately ID an individual. Some species are more vulnerable to overfishing than others, making species-specific information critical for conservation. Unfortunately, collecting those data typically requires trained scientists spending long hours at ports and landing sites, an approach that can be expensive and difficult to maintain. To address this problem, researchers tested a creative alternative: instead of relying entirely on scientists to monitor landings, they asked fishers to save the anal fins from every shark they caught. The fins were dried, collected and later analyzed by researchers. The approach essentially turns fishers into contributors to the “biological logbook.”
Over an 18-month period, fishers at two major fish markets in Puerto Rico submitted nearly 400 shark anal fins as part of a collaborative monitoring program. Researchers then combined fin shape analysis with DNA barcoding to identify which species were being landed, and used established relationships between fin measurements and body size to estimate the length of each shark. Altogether, nine shark species were recorded in the fishery; the most frequently encountered was the Caribbean sharpnose shark (Rhizoprionodon porosus), a species prohibited in U.S. federal waters, followed closely by the scalloped hammerhead (Sphyrna lewini), which is also federally prohibited and listed as ‘Threatened’ under the U.S. Endangered Species Act. Nearly all scalloped hammerheads were juveniles and none had reached maturity.
While it seems like a “small” detail, it actually is a big deal. The scalloped hammerhead is one of the ocean’s most recognizable sharks, defined by its wide, hammer-shaped head and unmistakable profile. But behind that iconic shape is a life history strategy that makes the species especially vulnerable to fishing pressure; these sharks grow slowly, mature late and produce relatively few young over their lifetime. Catching individuals before they have a chance to reproduce can have significant consequences for population recovery. “Unfortunately federally prohibited species making up such a large proportion of the catch isn’t surprising and [is in line with] our previous work,” said co-author Raimundo Espinoza Chirinos, Executive Director of Conservación ConCiencia, a non-profit organization in Puerto Rico dedicated to environmental research and conservation.
This pattern is closely tied to how shark fisheries are managed across jurisdictions. In U.S. federal waters, shark fishing is regulated under federal law, but within Puerto Rico’s territorial waters, management authority sits with local agencies. This split creates a situation where species prohibited under federal rules may still be legally landed if caught within territorial waters. As a result, more than three-quarters of the sharks documented in this study belonged to federally prohibited species. Now, this doesn’t imply illegal activity. In many cases, fishers are operating fully within the bounds of local regulations. But that might soon change. According to the team’s new DNA analyses, it suggests that Puerto Rico’s scalloped hammerheads are part of the ESA-listed Central and Southwest Atlantic population segment, rather than the Northwest Atlantic and Gulf of Mexico segment, which is not listed as threatened. “This means that continued landings of these sharks in territorial waters raise conservation concerns under the U.S. Endangered Species Act.” What’s even more striking is the fact that the Puerto Rican scalloped hammerheads were not only differentiated from the Northwest Atlantic population, but also showed genetic separation from populations sampled in Belize and Brazil! In other words, Puerto Rico may represent a unique and previously underappreciated component of scalloped hammerhead genetic diversity, one that the team hopes warrants targeted conservation attention. “As one of the first shark research organization in Puerto Rico, seeing the genetic findings that suggest our scalloped hammerheads are distinct and to date part of a unique Scalloped Hammerhead population is incredibly meaningful,” said Espinoza Chirinos. “It confirms the urgency of our local conservation efforts and highlights why establishing protections like the proposed Boca Vieja Scalloped Hammerhead Wildelife refuge is so vital for this specific population.”
“What stood out was how consistently the anal fin data reinforced that picture, particularly the dominance of federally prohibited species such as Caribbean sharpnose sharks and juvenile scalloped hammerheads. For the sharpnose species, the genetic results were also important because they showed that most of the catch was the Caribbean sharpnose shark rather than the Atlantic sharpnose shark. That matters because one is prohibited in federal waters while the other is not, so distinguishing between them is directly relevant for management,” said Kasana.
So what happens when conservation policies do not align with the realities of local fisheries? Blanket bans may seem like an obvious solution, yet fisheries management rarely operates in a world of simple answers and restricting certain catches could affect livelihoods, food security and community traditions. At the same time, failing to protect vulnerable species jeopardizes the future of shark a population we didn’t even realize was there to begin with. Thus, finding a path forward will likely require something more nuanced and include targeted solutions. For example, while Caribbean sharpnose sharks appear to be caught primarily as adults (potentially making them more resilient to fishing pressure), juvenile scalloped hammerheads represent a much greater sustainability concern due to their age. Understanding where and when these species overlap could help managers reduce hammerhead catches while minimizing impacts on fishers.
The researchers suggest continued collaboration among fishers, scientists and management agencies. The fin-monitoring program worked because fishers participated voluntarily. And while the method cost a fraction of traditional monitoring programs while still producing valuable scientific data, the research team doesn’t see anal fin monitoring as a replacement for traditional port-based surveys. “Direct observations provide valuable biological and fisheries information that fins alone cannot. Rather, we see it as a practical and scalable supplement that can fill important data gaps between more labor-intensive surveys,” stressed Kasana. As shark populations continue to face pressure across the Caribbean and beyond, the future of conservation may depend as much on human relationships as it does on shark biology itself.
