The Galapagos Sailfin Grouper: a Case Study in the Importance of Reproductive Biology in Fisheries Management
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© Daniel Cryan 2015 The Galapagos Sailfin Grouper: A Case Study in the Importance of Reproductive Biology in Fisheries Management Daniel Cryan Bill Durham Darwin, Evolution, and Galapagos 10/9/15 Cryan 2 Introduction The Galapagos Archipelago is world renowned for its unique collection of endemic coastal and terrestrial life. Here, one can find iconic species such as the Galapagos giant tortoise, the marine iguana, the flightless cormorant, and of course Darwin’s finches. Many visitors to the islands are unaware, however, that the waters surrounding the Islands are also home to their own diverse and fascinating collection of marine life. Here, a confluence of major currents unites species from the cool southern provinces of Chile and Argentina, with warm-water species from the west coast of Central America, as well as with tropical species from the Indo-Pacific. These waters are also home to an impressive number of endemic marine fishes, including the distinctive Galapagos sailfin grouper, or as it is locally known, the bacalao (Constant, 1992). The Galapagos sailfin grouper is an apex predator in the rocky reef ecosystems of the Galapagos Marine Reserve, and has supported a significant fishery in the islands since the early 1920’s. In recent years, however, fishermen have reported smaller and smaller catches of bacalao, in terms of both size and overall abundance. In this paper I postulate that the declines in the grouper population are largely the result of detrimental fishing practices that fail to account for (or even exploit) the unique reproductive biology of the sailfin grouper. I begin with a discussion of protogynous hermaphroditism in groupers, its role in reproduction, and the fisheries implications of targeting large males. I then examine the longevity and slow-maturation of the Galapagos grouper, and the potential consequence of targeting immature fish. Lastly, I discuss the role of spawning aggregations in grouper reproduction, and the importance of protecting key spawning sites from fisheries exploitation. Finally, I make a series of recommendations, based on my three hypotheses, on what conservation measures should be implemented to ensure the future of the Galapagos sailfin grouper. Cryan 3 Biological and Socioeconomic Background The bacalao or Mycteroperca olfax is a member of the subfamily Epinephelinae, a diverse group of fishes found worldwide, known colloquially as “groupers.” Taxonomists have recently reevaluated the phylogenetic lineage of this group of fishes, placing groupers alongside soapfishes and fairy basslets in the larger family Epinephelidae, separating them from the closely related anthias and sea basses, which now form their own family Serranidae. There are 16 genera and 163 species of groupers found worldwide, with 10 species and 6 genera found in the Galapagos Marine Reserve (Craig, 2007). Of these 10, the bacalao is the largest and most ecologically important grouper found in the Islands, growing to over 120 cm (4 ft.) long. Like most groupers, the Galapagos grouper has a large robust body with a wide truncate caudal fin. Its head and mouth are both large, and it is a fairly slow, deliberate swimmer. The sailfin grouper gets its name from its distinct dorsal fin, in which the second and third spines are elongated, giving it a signature sail-like curve. Most individuals are grayish brown to olive green in color, and often have light spotting and vertical markings along their sides, (though a small portion of the population, known locally as the bacalao rey or “king grouper,” are a bright yellowish gold color over their entire bodies) (Deloach, 2003). In most places, groupers are among the top predators in their ecosystem, and the Galapagos is no exception. Like its relatives worldwide, the Galapagos grouper feeds on a variety of crustaceans and small reef fish, and is only susceptible (as an adult) to species such as sharks or larger groupers. One study found that sailfin grouper fed mainly on the pacific creolefish (Paranthius coronus), another smaller grouper found throughout the islands. Like other large groupers, the bacalao is an ambush predator, spending most of its time cruising near the bottom, hiding near the mouths of caves and under ledges, waiting to strike out at Cryan 4 unsuspecting fish. Juveniles are often found in shallow mangrove forests and sandy lagoons, while adults tend to live in deeper waters around rocky reefs, drop offs, underwater lava ridges, and other vertical structures (Grove, 1997). Unlike many other groupers worldwide, who have grown wary of humans due to overfishing, the Galapagos grouper is still fairly gregarious (a trait common in species throughout the islands) and will sometimes approach divers or snorkelers with curiosity (Craig, 2011). An adult Galapagos grouper comes out from an underwater cave. © Daniel Cryan 2015 In the 1920’s, the first commercial fishery for Galapagos grouper began in the Archipelago. It was established by a group of Norwegian settlers who caught, dried, and salted the fish, so that they could be exported to mainland Ecuador. Many fish were used as the main ingredient in the traditional Ecuadorian dish Fanesca, a popular Easter stew. (Although this fact may seem trivial or irrelevant now, it will prove important later on when discussing the decline of the bacalao). This fishery not only marked the beginning of the exploitation of bacalao, it also heralded the beginning of the finfish fishery as a whole in the Galapagos (Lacquet, 2007). Over the next several years, more and more fishermen traveled to the relatively untouched waters of the islands to tap into the seemingly endless supply of fish. This growth coincided with the globalization of the Islands as a whole. Over the second half of the 20th Century, the number of tourists and residents in the islands increased exponentially, placing heightened pressures on the Cryan 5 Islands’ marine life. During the 1990’s, at the peak of the sea cucumber fishery, there was a record high of nearly 1200 registered Galapagos fishermen (Castrejón, 2013). Though bacalao is best known for its use in Fanesca, it is also delicious baked with a side of limes. © Daniel Cryan 2015 During this period of unprecedented growth, however, fishermen and researchers began to notice a change in the composition and size of the fishermen’s catch. In a reconstruction of the total catch from 1950 to 2010, Schiller et al. found that the composition of the catch had changed significantly over time. Serranids (the family containing the bacalao, as well as other groupers and sea basses) had once comprised more than two thirds of the total catch, but now they only accounted for about a quarter of the total, and had been replaced by species such as tunas and mullets (Schiller, 2013). Another study from Burbano et al. looked at the reported “best day’s catch” of Galapagos grouper amongst different age groups of fishermen. They found that older Cryan 6 fishermen used to catch significantly more Galapagos grouper than the younger generations, and that the fish caught a few decades ago were also larger (Burbano, 2014). These studies, along with the general absence of bacalao in fishermen’s catches nowadays, indicate that this species has experienced some level of decline over the past few decades. Determining the precise cause of this decline will be crucial for reversing this trend and ensuring the long term viability of this species. Reconstructed Galapagos fish catch from 1950 to 2010. © Schiller 2013. My Hypotheses When postulating reasons for the decline of a given species of food fish, it’s often dangerous to oversimplify the situation and assume that fishermen are simply catching too many fish. This fails to account for the nuances of the situation, and will inevitably lead to ineffective Cryan 7 or impractical conservation measures. A more practical approach is to examine the life history of a given species, and evaluate how fishing effort may influence a species’ ability to survive and reproduce. In the case of the Galapagos grouper, I propose that the majority of the decline over the past few decades can be directly attributed to fishing practices that work against the grouper’s unique reproductive biology. Specifically, fisherman have traditionally harvested large males, creating a skewed sex ratio. Nowadays, they target juveniles, before they have the ability to reproduce. And lastly, they often target grouper spawning aggregations, which once again threatens the population’s long-term viability. It is common practice for fishermen to target the largest fish possible. Larger fish equate to higher profits at market, and so it is in the best interest of a fishermen to catch the largest fish possible. As a result, in places where overfishing occurs, it is usually the largest fish that are the first to go, as long term survival becomes increasingly difficult. The Galapagos grouper is no exception to this rule. Fishermen are now catching increasingly smaller fish, as large individuals become extremely rare (Burbano, 2014). For many species this is just an unfortunate result of overfishing, and not a problem in and of itself, but for the Galapagos grouper, this lack of large individuals is especially problematic. This is because the bacalao, like many other groupers, is a protogynous hermaphrodite. A protogynous hermaphrodite is an organism that begins its life as a female, but later in life transitions to male, once it has reached a certain size (Bhandari, 2006). The evolutionary advantage of such an adaptation is that large healthy males are favored by natural selection and that there is a low likelihood of closely related species interbreeding, because at any given life stage they are all the same gender (Ghiselin, 1969). In 1993, Coello and Grimm conducted a study of the life history of Galapagos grouper, by examining the age, size, and sex of fish caught by Galapagos fishermen.