Tropical Seagrass

In the previous article I briefly outlined the complex life histories of most marine fish, including those we pursue with a fly rod, and summarized the importance of coastal environments to these fish – especially juveniles and young adults. (Note that a similar life cycle is followed by many of the species consumed as prey by recreational fish.) In this article I will address tropical seagrass beds that are not associated with mangroves (seagrass beds associated with mangroves will be the focus of the next article), and will consider bonefish, permit, tarpon, snappers, barracuda, and jacks to be the primary recreational fish species. Sub-tropical and warm-temperate seagrass habitats, which host some of the species listed above as well as red drum and speckled seatrout, will be the focus of a future article.

Tropical seagrass beds (primarily Turtle Grass, Thalassia testudinum, with some Manatee Grass, Syringodium filiforme) are usually located in shallow areas that are protected from excessive current and wave action. (Although Turtle Grass can grow as deep as 100 feet, it is usually found much shallower due to light requirements.) At first glance, these seagrass beds are lush underwater lawns with a limited number of organisms living among the blades of grass. But on closer inspection, an observant angler will see a habitat teeming with life. Since seagrass beds are shallow and very productive areas that provide food for many species, they are attractive areas for numerous small organisms, many of which are cryptic (well camouflaged to match their surroundings) and/or very adept at using the seagrass blades for shelter. Although seagrass habitat provides shelter for small organisms, such as shrimps, crabs, and small fish, there is inadequate shelter for large fish (as might be found on a reef), which is one reason large fish often forage in shallow seagrass beds, but retreat to deeper water when they feel threatened.

One important ecological function of the broad blades of Turtle Grass is to reduce the velocity of the currents that flow over grass beds so that sediment particles suspended in the water by these currents drop to the bottom.  This acts as a filter, keeping sediments from reaching reefs, and provides food for numerous organisms that feed on the microorganisms attached to the sediments in seagrass beds and on decaying plant and animal matter (this collection of sediments, plant, and animal matter is called detritus).  In addition, Turtle Grass supports a diverse array of algae and invertebrates that attach to the surface of the grass blades (the organisms attached to the surface of the grass blades are called epiphytes).  In turn, these organisms fall prey to grazers and predators, and in this way the productivity of the seagrass beds supports the food web that brings larger recreational fish into these shallow waters.  In over-simplified form, the food chain can be summarized as: sunlight and nutrients support Turtle Grass±sediment and detritus, epiphytic invertebrates and algae (also use sunlight) + invertebrates and small fishes (feed on small invertebrates and graze on algae and seagrass blades) + larger (i.e., recreational) fishes. 

Tropical seagrass beds support an incredibly high diversity of species of fish, crabs, shrimp, copepods, amphipods, worms (a broad category including many different families), algae, echinoderms (urchins and seastars), and a host of other groups of organisms.  Although only a few of the fish species are targeted by recreational anglers, many of the fish and other organisms are potential prey items for recreational fish.  It is importance to understand that the abundance of species, or even the collection of species found in a seagrass bed may change among locations, so although the general list of prey items available to recreational fish may be similar, the favored prey items will likely vary.  This, in part, explains why some flies are pounced upon by fish on one seagrass flat, but completely ignored by fish in other locations.  In addition, abundance and presence of potential prey items for recreational fish changes seasonally due to factors such as temperature and life cycles of these species (See the previous article for a description of how the life cycles of marine fish and other species influences seasonal abundance.) 

So although you might wish for a list of prey species you can expect to find on a tropical seagrass bed, that list will depend on where you will be fishing and even on the time of year.  However, we can use our knowledge of the general makeup of communities found in tropical seagrass beds to generate a list of types of prey items that will most likely be present on any seagrass beds you might fish, and give you a head start on deciding which flies to put in your fly box. 

Although seagrass beds are home to many species that are fed upon by recreational fish, the structure of the seagrass habitat makes it almost impossible to fish flies that imitate many of these prey species.  For example, most worms and urchins move slowly (or not at all) along the bottom of the grassbeds, so are difficult to imitate with a fly in areas of thick seagrass.  However, grassbeds with sparse grass, and areas of exposed bottom, may be amenable to such non-traditional patterns as feather-duster worms and urchins.

In his book Fly Fishing for Bonefish, Dick Brown summarizes the findings of a number of scientific studies that examined the availability of prey, food preference, and overall diets of bonefish found in shallow water habitats, including seagrass, in Puerto Rico, Florida, and the Bahamas.  In each of these locations, the types and abundances of prey species varied, as did bonefish prey preferences and diets.  For example, most Florida bonefish had crustaceans in their stomachs, while clams were most frequent in stomachs of bonefish from Puerto Rico.  I found similar differences in prey preferences for permit in the Virgin Islands.  On St. Croix, U.S. Virgin Islands, where permit come into shallow seagrass beds that are mixed with coral rubble to feed, analysis of stomach contents revealed a preference for small clams and small sea urchins.  At locations in the nearby British Virgin Islands, where permit are most often found in shallow seagrass beds that are intermixed with sand flats, crabs and shrimp are high on the permit menu.  These differences in diets of bonefish and permit among locations is most likely due to differences in the abundances of prey items. 

Although the number and types of species may vary among locations, most tropical seagrass beds have members of particular families living among the grass blades.  We can use this information to narrow the list of probable prey items we might find on a particular seagrass bed.  For example, crustaceans (primarily crabs and shrimp) are usually abundant in seagrass beds.  An important characteristic of these crabs and shrimp is that although the species may vary, the general colorations and behaviors can be strikingly similar.  For example, both swimming crabs (Portunidae) and walking crabs (Goneplacidae), probably the most common small crabs on tropical seagrass beds, will scurry for cover rather than try to outrun a pursuing fish, but they will generally differ in where they can be found.  Swimming crabs will usually be found in seagrass beds with soft or sandy sediments and usually bury in the sediment as a defense strategy, while walking crabs prefer seagrass beds with some rocky habitat mixed in and will scurry into the nearest hole or under the nearest rock.  Crabs from both families will be colored so they are well-camouflaged in their surroundings – whether green in areas of thick Turtle Grass or mixed tan and green for areas where coral rubble is mixed with the seagrass. 

Shrimp will also take on the coloration of their surroundings, although some – like the grass shrimp – may be mostly clear, and just like crabs the species will vary depending upon location. The most common species of shrimp found in tropical seagrass beds are members of three families: common shrimp (Pinnaeidae) are what we usually think of when we think of shrimp, and are usually found in the same areas as swimming crabs; mantis shrimp (Squillidae) will live in holes among coral rubble or will live in burrows in soft-bottomed seagrass beds; snapping shrimp (Alpheidae) are usually found among rubble or empty shells within seagrass beds; and grass shrimp (Palaemonidae) live on and among the grass blades.  Depending upon the species and their habitat, in order to escape from predators the shrimp may burrow into the sediments, scurry under rocks or into burrows. hide among the bases of the grass blades, or grab onto seagrass blades and try to become ‘part of the seagrass’.

Even in areas where crustaceans might not be the most common food item, such as on St. Croix where clams are a favorite for permit, flies imitating crabs and shrimp should be high on your list of go-to flies.  Crustaceans arguably pack the greatest reward for the effort for recreational fish because they are very high in caloric content for their size, so even if they are not the leading prey item crustaceans are almost always on the list of prey items found in stomachs of recreational fish in tropical seagrass beds.  Most importantly, crustaceans can be imitated with a fly.  Even though bonefish in a particular location may eat a lot of small clams, a clam fly isn’t really feasible, but a small shrimp or crab that looks similar to a local species may be the next best choice.

Tropical seagrass beds are also home to dozens of species of fish.  For example, in a recent survey of a seagrass bed in the Caribbean, 91 species of fish were recorded.  Some of these species are present year round, such as gobies, blennies, and some species of wrasse and parrotfish.  Excluding the larger recreational fish that were recorded in the survey, most individuals of the fish species are relatively small (generally less 4"), are fed upon by numerous recreational fish species throughout the year on an opportunistic basis, and generally comprise a relatively small but important portion of the diet. 

Other species are present seasonally, such as juveniles of many larger species that inhabit reefs and deeper water as adults.  Recreationally important species that use shallow seagrass habitat as juveniles include snappers, permit, and barracuda.  Other species that can be seasonally abundant as juveniles include grunts, surgeonfish, wrasses, and parrotfish.  During summer months, these juveniles can be important food items for larger recreational fish, especially barracuda, jacks, snappers, and even bonefish.  However, the abundance of these juveniles changes from year to year, so what may have been a favorite food item in one year might not be very common in the next year.

An important aspect of imitating the fish that inhabit seagrass beds with a fly is the fish’s behavior.  If you ever go diving or snorkeling over a seagrass bed, you may see fishes swimming just above the tips of the grass blades.  As you approach these fish, they will either dive into the grass blades for cover or speed quickly away, skimming the tips of the grass blades as they flee in a zig-zag pattern.  The latter behavior is one that is most amenable to imitation in the design and fishing of flies in these areas.  For example, yellowtail snapper use seagrass beds as nurseries, and can be especially abundant in summer.  A yellow and white fly of 3" - 5", fished just above the grass blades is a good way to imitate these juvenile fish.

There are also notable day-night changes in the activities of many species found in seagrass beds.  This is especially true for many of the crustaceans, many of which are far more active at night.  This is why dusk and dawn can be great times to target large fish feeding in shallow water.  Dusk and dawn are the change-over times for the day and night groups of species, so fish feeding during these times have access to prey that is both coming out to feed and returning to shelter.  So although many recreational species can be found feeding throughout the day, their most active feeding times might be when light is lowest.  For example, on Caribbean islands, large snappers will often forage in shallow seagrass beds at night, and can be targeted with a fly rod at both dawn and dusk.  Bonefish and permit can be found searching for small swimming crabs along shallow seagrass shorelines at dawn and dusk. 

Although this discussion has focused on the benthic organisms that rely directly on seagrass habitat, seagrass lagoons are also home to numerous fish that remain in the mid- and upper-levels of the water column, such as numerous species of herrings (Clupeidae) and silversides (Atherinidae).  Seagrass lagoons are attractive to these species because these areas are relatively shallow, sheltered, and calm, and are not as heavily populated by predators as nearby reefs.  These species tend to school during the day and disperse at night to feed.  Although their abundance can vary seasonally, these species are generally present throughout the year, and during the daytime hours, these species are preyed upon by virtually all larger fish species, so you should carry an abundance of flies imitating these baitfish (from 1" to 6") . 

In closing, the extent to which the communities found in seagrass beds depend on the food and shelter that can only be provided by a healthy environment cannot be overstated.  Boat anchors, boat propellers, damage from boat hulls from groundings, sediments and nutrients from land runoff and development, sewage, and drilling muds all negatively impact seagrass beds throughout the tropics.  Seagrass beds that are damaged by these actions lose some of their ecological integrity, and are unable to support full and diverse communities.  Therefore, the health of these ecosystems is paramount to ensuring a productive recreational fishery, so fish these areas responsibly.

Sidebar 1 – In general, areas of thick turtle grass will have a higher diversity of species (i.e., more total species) and a greater number of individuals than areas of sparse turtle grass.  (In other words, greater density of seagrass blades usually means a greater density of potential prey items).  However, the thick grass blades also provide better shelter (more places to hide and escape predators) for prey species, so the feeding efficiency of predatory fish is often lower in areas of thick seagrass than in areas of sparse seagrass.  This will be especially true during the day, when many prey species (e.g., shrimp) are usually deep within the seagrass.  In contrast, areas of sparse seagrass provide some shelter for small organisms but also provide a situation where the predatory fish have a better chance of spotting and then successfully pursuing and catching prey.

The density of seagrass blades is also an important factor when it comes to fly fishing.  Remember that the usual strategy for crabs and shrimp to avoid a predator is to perhaps make a quick, darting move, and then bury in the bottom sediments or scurry into a hole or under grass blades.  This means that flies that imitate crabs and shrimp are normally fished on or near the bottom.  This will effect what flies to choose and how to fish them in different types of seagrass beds.  For example, a shrimp-imitating fly that is fished near the bottom is more likely to be spotted and taken by a fish if it is in an area of sparse seagrass.  In thick seagrass, the fly may become lost in the seagrass and never spotted.  In general, when fishing shallow areas, I like to fish lightly weighted or unweighted flies in areas with thick seagrass, and lightly- or medium-weighted flies in areas of sparse seagrass.  Not only are weighted flies more likely to become lost in thick seagrass and never seen by the fish, they are also more likely to snag blades of seagrass.  Also, tying fly patterns in the bend back style, or simply with the wing tied in so it covers the hook point (like a Fernandez snapping shrimp) makes flies semi-weedless.  In deeper areas of thick seagrass I like to use heavily weighted flies if I want to get the fly to the bottom quickly, but if I am sight-fishing I make sure to cast the fly close enough to a spotted fish that it sees the fly sink to the bottom.  Otherwise, the fly will surely get lost in the thick seagrass.

In areas with thick seagrass I like to cast streamers (small clouser minnows are a favorite - either with beadchain or lead eyes, depending on the water depth) and retrieve the streamers with quick, short strips so the fly rides just above the tips of the grass blades.  This is a good imitation of the behavior of many of the small fish that live in turtle grass beds.  In areas with schools of small baitfish, retrieving the fly so it rides near the surface give you a good shot at jacks and small barracuda.

Sidebar 2 – Some of the best spots to target in turtle grass beds are where there is a change in the thickness of grass blades – a noticeable contrast between an area of thick seagrass and an area of sparse seagrass.  Predatory fish will cruise along the edges where these two areas meet, often zig-zagging across the boundary into both areas.  This situation will give you a chance to present a fly to a fish over sparse seagrass, which will give you a more varied choice of flies (unweighted vs. weighted; or bottom flies such as crabs vs. streamers) and style of retrieve (let the fly drop to the bottom (e.g., crab fly) vs. quick strips (e.g., baitfish streamer).  Areas of thick seagrass that change abruptly to sand or very light seagrass present similar opportunities, and even more options with fly selection. 

Many of the prey species you will encounter in turtle grass beds will have a coloration similar to their surroundings.  In this light, you may want to carry a couple of color variations of some of your favorite patterns.  But if you are limited in how many flies you carry, or find yourself in a situation like that described above (fishing in a mixture of bottom types), I’ve found that crab and shrimp flies in tan perform well in most habitats.