Larval Fish

---

A filefish finds temporary security next to our strobe.

The irony certainly wasn't lost on me as I sat perched on the gunnel of the Blenny watching five friends bring in fish after fish. We were moored on a calm night sea just off Bimini, the acclaimed gamefish capital of the Bahamas, whose legendary waters have, for decades, produced trophy marlins and tunas for trophy egos. Our little band wasn't after the big stuff; instead, we intently worked the opposite end of the spectrum, dipping up tiny bits of fishlife with aquarium-sized nets, duct-taped to mop handles.

Each larval fish was quickly deposited in an aerated pail so no one would miss a chance to snag other drifting specks attracted to the two night lights that bobbed just off the dive platform. After a frenzied hour, when the pickings became slim, we turned our attention to the exotic assortment of fish shapes that buzzed inside the bucket. Thin, translucent eellike creatures wiggled wildly near the surface, while others, displaying polished silver sheens, slowly swam the circumference or rested passively on the bottom. For us, each little wonder added a new dimension to our growing understanding of reef fish ecology.

A member of the jack family in its juvenile pelagic stage.

My interest in larval fish began the previous summer. While returning to the boat after a dawn dive, I spotted two of the smallest fish I had ever seen hovering next to a down line. Through my camera's viewfinder, with the macro lens focused one-to-one, I made out the unmistakable shapes of a miniature trunkfish and triggerfish, each no larger than a jelly bean.

After returning to the States, I phoned Dr. James Bohnsack at the National Marine Fisheries Center in Miami and mentioned the fish. Without hesitation he explained that the two little fellows I had seen were settling reef fish, the final phase of an extended planktonic larval existence.

"You mean these are the offspring of fish we see spawning in Bimini?" I asked.

"It's not likely they were spawned in Bimini. The fish you saw probably started life weeks earlier, possibly hundreds of kilometers away, and only because of a complex combination of events, many of which we are only beginning to understand, happened to settle in the Bahamas."

"I'm confused. You're telling me that the fish now inhabiting Bimini's reefs were not produced by the native population, but drift in from somewhere else?"

No bigger than jellybeans, tiny triggerfish and trunkfish hover next to our boat's downline before settling.

"Well, not drift exactly; the phenomena is not completely passive, but the chances are that they were spawned somewhere else. Until a few years ago it was widely held that the amount of food and protective nitches on a reef limited the number of fish and species that inhabit a particular site. Now, it is generally believed that tides, winds, currents and topography combine to carry most offspring far from their origins. When they've developed sufficiently, the fish settle wherever they happen to find suitable bottom. It's what we refer to as an 'open system.' Every reef fish family in the Caribbean, except toadfishes, have a pelagic phase. That's why toadfish are restricted to the continental shelf and never inhabit oceanic islands."

"Why don't I send you a few papers, and, in the mean time, I suggest that you read a book by Peter Sale entitled, The Ecology of Fishes on Coral Reefs. The book does a good job summarizing what is presently known about the life cycle of reef fish. Much of the text deals with their larval stage."

A week later the compact seven hundred page volume arrived. That evening, with book in tow, I retired early to the screen porch. Even before finishing the third paragraph, I was up and out of the hammock searching for a dictionary and highlighter. This was not an easy read, but, after struggling through a few chapters, I became hooked. A few weeks later, when I finished the text, my long held ideas about reef fish population biology had been radically changed.

Like almost every fishwatcher or marine biologist, I had also believed the classical "niche theory" -- the dogma of the day that reigned supreme among reef fish ecologists throughout the fifties, sixties, and well into the seventies. Its basic premise -- competition for limited food and space resources, coupled with predation, governed the local abundance of fish species. It further theorized that reef fish communities were "closed" systems where spawned offspring repopulated their natal reefs.

A settling flying gurnard. For a young fish, the ocean is fraught with danger, and only a very few and the very lucky survive.

Thinking back, it is easy to understand how everyone was seduced. The niche theory seems to make such good sense when observing a reef fish community in action. To my unsophisticated eyes, there appeared to be a never-ending turf war taking place. Every niche in a reef's craggy face seemed to house an animal ready to protect itself, its territory and its food supply. I simply assumed that the local offspring of spawning reef fish quickly found protected corners in their home reef, or were eaten or driven away.

Unfortunately, ecologists were so enamored with their pet theory that nearly everyone failed to take note that marine fisheries management had, for decades, been working with a different population model that focused on the relationship between adult stock size and larval recruits that arrived from the pelagic realm. As early as 1914, European fisheries biologist, J. Hjort, indicated the potential importance of recruitment to adult populations of marine organisms. In 1950, G. Thorson stated that marine invertebrates have a planktonic phase and a "settled" juvenile phase. However, these and other early insights were, inexplicably, absent from the favored ecological theory.

A tiny settling butterflyfish, no larger than your little fingernail, bravely traverses the open sand flat.

During the late seventies and early eighties, when evidence supporting an open system could no longer be ignored, marine ecologists scrambled to get on the bandwagon even though the new hypothesis was, and remains, difficult to demonstrate. Although pelagic field studies are logistical maelstroms, that, more often than not, produce perplexing sets of variations, ecologists have made great strides, over the past 20 years, improving their larval collection methods and gaining a better understanding of physical oceanography. Aiding everyone's efforts was the development of the daily otolith aging technique discovered by Panella in 1971. This significant new tool makes it possible to age a fish from embryo to adult by counting concentric rings of calcium carbonate that appear similar to growth rings of a tree. Otoliths are found within the semicircular canal of all bony fishes. Daily increments are distinct, and easy to read, on larval and juvenile fish. The technique provides a wealth of indispensable knowledge, including the duration of a fish's planktonic larval stage, and even backdating to determine the exact day when a fish was spawned.

Anna and I have, for the past two years, observed the spawning behaviors of many reef fish species. Nearly all strategies place gametes high in the water column, near dusk, when the eyes of predators are least effective. This hedge against the ever hungry mouths that encircle a reef is the only assistance offspring receive before being cast off on one of the most dangerous odysseys in the animal kingdom. If one considers that each living adult only needs to successfully reproduce itself once or twice during a lifetime to stabilize species numbers; and, that annual egg production from an individual often numbers in the tens of thousands, then it's easy to comprehend the appalling odds against the survival of larval fish in the vast pelagic orphanage.

The settling surgeonfish is easier to recognize than many juveniles, as it already has its distinguishing characteristics.

Within 24 hours, fertilized eggs hatch into tiny fish with long tails and distended bellies. These yolk-sac larvae, that alternate their time between long vigorous swims and rest, are high-energy eating machines that must locate a concentrated food source within two days or starve. Fortunately, the ocean's surface transports random clouds of food, known as patches. If these patches didn't exist, food particles would be so widely distributed that larvae of all types would perish.

In tropical waters, patches begin on warm, calm days when an upwelling brings cool water barreling up from deep below. The sudden mixture of nutrient-rich water and sunlight creates a wild bloom of phytoplankton, the soul food of copepods, the most abundant group of animals on earth. These tiny pelagic crustaceans begin to gorge and quickly multiply by the millions. Soon, macro-mollusks, salp, larvaceans, a distant relative of tunicates, and other pelagic herbivores add their biomass to the ever-thickening biological soup. Then, to the banquet, come the carnivores whose numbers are dominated by elongate arrow worms with formidable mouths ringed with teeth. They are joined by jelly-plankton trailing deadly mops of stinging tentacles. Fast-swimming juvenile and adult pelagic fishes constantly cut through the moveable feast, taking what they can. Even though this savage pelagic world should seem a nightmare to our cherubic little yolk-sac fish, its instincts indicate otherwise. For the next few weeks, this drifting patch of snapping-jaw hell will be home-sweet-home stocked with a full cupboard of food only a tail thrust away.

In the beginning, young fish larvae consume a variety of food, including phytoplankton, larval copepods and mollusks, and, in some cases, siblings. As they mature, their body mass increases by a third each day and each species begins to select a more specialized diet.

Fortunately, evolution didn't forsake the little fish during these perilous times. As the yolk-sac disappears, larvae develop elaborate morphological adaptations that attain a state of complexity that delights the imagination. Spikes, spines and spinelets, streaming fins bristling with barbs, bulbous heads sporting enormous jaws, and stalked eyes are a few adaptive disguises designed to deceive predators. Silver or transparent bodies aid in their death-defying game of hide-and-seek.

Drifting food patches vary horizontally from meters to kilometers, depending on the stability of the surface waters. When storms brew, wind and waves break patches apart; during periods of calm, they retain their size or may even grow when they occasionally coalesce with other patches. Nearly all larval fish inhabit the upper 100 meters where different families prefer different depth ranges during the day. At night, these strata often lose definition, when larvae tend to move toward the surface.

Otolith increment counts have established that a typical oceanic larval voyage in the Caribbean lasts between 14 and 30 days. When ready to settle, some larvae undergo a second dramatic morphological shift, while others change little more than color. The queen triggerfish, which spends 75 days in the pelagic environment, is one of the largest species at settlement, measuring 60 mm; the smallest, a reef drum, known as the cubbyu, is only about 4 mm when it ends its pelagic existence after 14 days. Often, larvae mature into their settlement mode days or even weeks before locating suitable settlement habitat; others, that are never swept near shore, drift aimlessly until death.

The pelagic larvae's final destination is dictated by a complex suite of hydrological and meteorological events and the influences of local topography. Even though there are some similarities, from year to year, indicating when and how many larvae arrive at a given reef system, for the most part, recruitment predictions are haphazard at best. If, during a given year, a high or low influx of a less common species occurs, it may affect the local population for a decade. Although there must be governing dynamics at work, the composition of a reef fish community seems the result of a great larval lottery influenced by the caprice of nature.

Most reef fish larvae settle on high profile reefs, however; others require intermediate habitats, such as sand flats, shallow patch reefs, grass beds, mangroves, or surf zones. For example, Nassau groupers almost exclusively settle in algal-covered clumps of dead finger coral located inside shallow mangrove lagoons. Settlement-stage fishes have well-developed senses, including sight, taste and smell, as well as significant behavioral and swimming abilities, yet how they are used to locate suitable habitat remains a mystery.

Near land, patches often join together creating long narrow fronts that can extend for over two kilometers. Recruitment reaches its peak, during summer nights, around the time of a new moon, when pulses of mature larvae surf toward shore on the flood tide. There, under the cover of darkness, when predation is least, they drop quickly to the bottom. The wrasse and eel larvae immediately burrow into the safety of the sand. Others, it is believed, hide inside the reef or deep within rubble fields, where, during the following days, they develop into juveniles that must face a new way of life every bit as dangerous as the open sea. On Caribbean reefs, where piscivores often make up a large percentage of the biomass, it is estimated that juvenile grunts experience 90 per cent mortality during their first month; for the year, the rate rockets to 99.9 per percent.

On the second evening of our five-day larval fish expedition, our group spent the night on the Blenny, so that just before dawn we would be able to check the web of weighted lines that were hung below in the hope of attracting settling fish during the night. Little is known about where most fish hide when they first leave the pelagic environment. Our plan was to first photograph the fish, then lift the lines, one at a time, forcing them to settle as an observer followed. With visions of little fish flocking to our dangling down lines, we spread blankets wherever we could, and drifted off to sleep. At dawn we rose, stiff and sore, to a ripping current that would have blown any interested larvae far away and made diving impractical.

On the third evening, instead of larval fish, our night lights attracted a living tornado of minnowlike fish that madly spiraled from night lights to the sea floor 35 feet below. On the morning of day four, the National Weather Bureau in Miami informed us of Hurricane Bertha. The storm, that passed a hundred miles east, turned our lovely water into chalk. We were quickly learning about the thousand-and-one demons that bedevil field researchers working on the open sea.

Disappointed, but undeterred, we traded tanks for snorkels and, for the final two days, explored the seagrass meadows and mangrove thickets inside the calm waters of Bimini Bay. There, hiding among grass blades and roots, we discovered a different cast of recently settled characters, including tiny barracudas, as thin as pencil lead, dime-sized beaugregories and sergeant majors, and young gray and schoolmaster snappers by the hundreds. From these rich nursery grounds the magical little fish will, when ready, migrate to nearshore patch reefs, and later to a final home in a deepwater reef habitat.

My first thought, after learning that reef fish communities function as open systems, was of the model's implications for marine wildlife reserves -- a cause close to our hearts. At first reading, it seems a potentially powerful card that will play well in the hands of reserve opponents. Commercial and recreational fishermen have long enjoyed a tradition of virtually unrestricted access to the sea's bounty. Why should they now allow large reef tracts to be set aside as no-harvest zones if the benefits simply float off to someone else's reefs? Their vocal opposition has, over the years, limited the establishment of marine reserves around the world, and was recently responsible for severely restricting the size of such zones in the new management plan for the Florida Keys National Marine Sanctuary.

It is well documented that the larger individuals of a species' breeding population are hundreds of times more prolific than smaller individuals just entering sexual maturity. For instance, a single 25 pound red snapper can produce nine million eggs per spawn; it would take 212 smaller snapper to generate an equivalent number. Yet, these same great, genetically superior fish are the traditional targets of harvesters. Large, protected, and properly administered marine reserves, extending from inshore habitats to offshore depths, would not only afford sanctuary for the "ol' mammas and papas", but would also allow a full complement of sea life to live together unmolested by spear, hook or net.

In early October 1996, Anna and I attended an open forum, in Gainesville, Florida, to discuss the future of marine reserves. For a day and a half, one hundred attendees representing fisheries management, the National Marine Sanctuary, environmental organizations, enforcement, education, media, commercial and recreational fishing interests, and divers presented their varying views on the subject. To say the least, a consensus was not reached.

Of course, selfishness is the central tenet of each user group. Commercial fishermen, still stinging from a recently imposed net-ban, appear more determined than ever to limit further governmental regulations on their livelihood. The biggest surprise, however, was the seemingly intractable resistance to no-harvest reserves from panelist Doug Kelley, managing editor of Florida Sportsman magazine, who supposedly represented the state's recreational fishermen.

Divers, who didn't have a delegated spokesperson, managed, nevertheless, to have their concerns heard. They argued that it is unfair for harvesters to have free range over all continental waters, especially coral reefs. They felt that large areas, much like our National Park system, should be set aside in the ocean for the protection of marine ecosystems. To this Doug Kelly responded that it was inequitable for divers to gain a benefit at the expense of fishermen. If no-harvest reserves are established, divers should also be excluded. He continued by stating, "If the whole purpose is just to see sea life, go to the Seaquarium.", and, "You're crazy, if you don't think divers and their big cameras scare away fish." and, "Why should we have reserves if there is no data to conclusively support their possible benefits?"

Commercial fishermen tend to be an independent lot with little patience for rules, regulations or paper work. As fishing methods mechanize, competition multiplies, regulations encroach and fish stocks plummet, they become more defensive. On two occasions, the first on an eastern Caribbean island, and, the second in Florida, I heard two different commercial fishermen make almost identical statements: "My grandfather fished, my father fished, and by God, no one is going to stop me from fishing." When asked, "What about your sons?" they both voiced similar sentiments. "My sons will have to take care of themselves."

I believe that, in their heart of hearts, both commercial and recreational fishermen realize that fisheries reserves are not only inevitable, but an urgent necessity to protect their long-term interest. They, more than any other group, have witnessed the sharp decline of fish stocks in recent decades. No-harvest reserves, which have proven to increase the local egg production of Naussau grouper five to seven fold, appear to be the best solution for their ever-worsening situation. If they continue to overfish the already faltering breeding stocks, fisheries will collapse. If this occurs, it could be a long time, if ever, before the species makes a comeback to sustainable numbers. However, with wise fisheries' management, and the establishment of breeding reserves, there is a possibility that limited-yield fishing could continue indefinitely.

But what about marine fishes that do not have direct commercial value? What about the damselfishes, the trunkfishes, the gobies, blennies and hundreds of other irreplaceable fishes? It is hard to believe that their fate is not intrinsically linked to gamefish and market fish, as well as all other creatures on the reef. The establishment of no-harvest coral reef wilderness reserves, would not only be a boon for reef fish, but also divers, who, more than any other group, take pleasure from the spectacle of a bountiful reef community. But for underwater sightseers to be excluded just because harvesters are excluded is a childish, tit-for-tat ruse. Without question, the diving community has an inherent responsibility to lead in the promotion, establishment and maintenance of marine reserves. A yearly diver-users fee, such as the one administered in Bonaire, that goes directly for management and enforcement of reserves, would be a pragmatic first step, demonstrating our commitment to the well-being of coral reef ecosystems.

The number of fish larvae that are carried off to distant reef systems, often across international boundaries, and the number caught in gyres and spun back to the reefs of origin, remains unknown. But, in the long run, are these numbers what really matters? Our overriding goal should be the protection of marine life, not the protection of interest group politics, pocket books, and egos. If we are forced to wait for indisputable proof that marine reserves will work, on all levels, for all people, it will be too late for everyone. It is a time for soul-searching.

Lying on the sand, squinting at a recently settled, silver, black and transparent reef drum, not much larger than one of the coarse grains of coral sand that it relentlessly swims back and forth above, I now know something of the great dangers it recently passed through; I also know the odds against its continued survival. I do not say anything to the fish about the things I know.

© 1999 New World Publications. All rights reserved.