[navigation information]

WHERE ARE ALL THE CORAL TROUT ?

Tony Ayling and Avril Ayling

This publication may be referred to as:

Table of Contents.

List of Figures.

Figure 1.
Length frequency distribution of the two colour forms of the footballer/blue-spot coral trout showing the differences in size.
Figure 2.
Changes in numbers of the different coral trout species across the continental shelf from turbid coastal reefs to the front of outer shelf reefs washed by the clear waters of the Coral Sea.
Figure 3.
Numbers of the common coral trout on mid-shelf reefs in four major regions along the length of the GBR.
Figure 4.
Numbers of the footballer/blue-spot coral trout on outer shelf reefs in three major regions along the length of the GBR.

WHERE ARE ALL THE CORAL TROUT ?

Tony Ayling and Avril Ayling

Everyone wants to know where all the coral trout are. Recreational fishermen want to know so that they can catch enough for a feed and add the perfect touch to that day on the water, commercial fishermen want to know so they can make a good living and the Great Barrier Reef Marine Park Authority want to know so that they can manage the resources of the Great Barrier Reef (GBR) more effectively. We have spent hundreds of days at sea over the past few years trying to find out for the Marine Park Authority just how many coral trout there are on the GBR and where they live.

The Marine Park Authority held a number of workshops in the late seventies and early eighties to develop techniques for counting coral trout. We used a method that involved two divers searching for coral trout along 50 metre long by 20 metre wide survey transects (1000 square metres). To cover as big a depth range as possible we ran the 50 metre fiberglass surveyors tape that defined each transect down the reef slope from the edge of the reef flat. On most reefs the transect ended in about 15 metres of water but on some shallow reefs the end of the tape was only at 8 or 10 metres depth and on the steep slopes of some outer reefs we reached depths between 20 and 30 metres. We surveyed ten separate transects on each reef scattered along about a kilometre length of reef edge.

But where on each reef was the best place to make our counts? Obviously we could not cover the entire reef with only ten transects twenty metres wide. We made surveys in a variety of habitats on a few reefs to see where most coral trout were found on a reef. There were very few coral trout on the reef flat or in shallow lagoon areas compared to the reef slopes. On the reef slopes we found that there were fewer coral trout on the exposed windward or front side of the reef, facing toward the predominant southeast wind, than on the more sheltered leeward or back reef. On average there were about 40% more coral trout on the back reef than on the front so we decided to confine our counts to the back reef slope habitat. This also has the advantage of being a safer place to work when strong southeasterly trade winds are blowing.

GBR Survey of Coral Trout

To cover the entire GBR region we had to make some other decisions. Clearly we could not count coral trout on all the 2,500 separate reefs the Marine Park Authority has listed. After consultations with the Marine Park Authority we chose reefs to survey at a wide range of positions from Triangle Reef at the top of Cape York, with a latitude of 10°30' south, to Lady Musgrave Island in the Capricorn-Bunker Group at a latitude of 24° south. The selected reefs also ranged across the shelf, from turbid coastal reefs where even seeing the coral trout was a problem, out to the reefs of the outer barrier rampart on the edge of the continental shelf where visibility was often over 30 metres. We surveyed a total of 156 different reefs over a three year period, spending more than 200 days at sea. The sea time was spread over four major field trips on large charter boats, spending up to 50 days at a time at sea.

Different Coral Trout Species

As most fishermen know there are actually several species of coral trout. All of them are fish eating predators and they are the most important reef-living predators on the GBR. They have a life cycle that sees all individuals start life as females and spend one or two years reproducing as a female before changing sex to become a male. All large coral trout are males. They are rapid growing fishes, becoming reproductively mature in their second year and reaching the 35 cm legal minimum catchable length in one to two years.

We started off counting five species. The common coral trout, known by the scientific name of Plectropomus leopardus and sometimes called the leopard trout, was the one most often seen. It ranges from white-grey to green-grey to red-brown, and is covered with small blue spots with a distinctive blue eyebrow. On turbid inshore and coastal reefs we encountered the bar-cheeked coral trout Plectropomus maculatus, a species that was usually orange brown in colour with larger, bright blue spots, and those spots on the side of the head elongated into bars. Occasionally we would see a footballer coral trout Plectropomus laevis (the scientific name Plectropomus melanoleucas is sometimes used for this species), sometimes known as the tiger trout, with its distinctive colour pattern. The five black saddle markings with outlines of blue dots on a blue-spotted white body and bright yellow fins make this species unmistakable. On outer barrier reefs the blue-spot coral trout was common. This species apparently had not been given a scientific name and was known as Plectropomus sp. The blue-spot reached a larger size than the other coral trout, we saw individuals up to 120 cm long, compared to the around 65-70 cm maximum of all the other species. Blue-spot coral trout are red-brown with darker brown saddles and a scattering of widely spaced large blue spots on the body. A rare species that we only saw a few times was the passionfruit coral trout Plectropomus areolatus. The colour pattern of this species incorporates numerous large blue spots set close together over the entire body; the other species do not have blue spots on the belly.

We discovered part-way through the project that the footballer coral trout and the blue-spot coral trout were in fact dramatically different colour forms of the same species, with the scientific name Plectropomus laevis. At first we doubted that this could be true, but as we thought about it the pieces fell into place. We had never seen any young blue-spot trout, the smallest we had seen were about 20 cm long. The reason for this is that all blue-spot coral trout start life in the footballer colour pattern and later change colour to the blue-spot pattern; all individuals over about 65 cm long were in the blue-spot pattern.

Why does this species have two such different colour patterns? The reason for this became obvious when we saw some very small footballer coral trout and observed their behaviour. The footballer colour pattern with its black saddles, blue spots and yellow fins is almost identical to that of a small pufferfish, the black-saddled toby Canthigaster valentini. This pufferfish is highly toxic and is recognised and avoided by predatory fishes.

Figure 1.  Length frequency distribution of the two colour forms of the <BR>footballer/blue-spot coral trout showing the differences in size.

There is a small leatherjacket that mimics the pufferfish colour pattern so that predators mistake it for a toxic pufferfish and do not eat it, and it appears that the small footballer coral trout are attempting the same mimicry, with some behavioural traits that enhance the similarity. Unlike juveniles of the other coral trout that swim with body undulations like the adults, juvenile footballers hold their body rigid, fold their tail to resemble the puffer, and swim with pectoral fin beats. They also hold the front half of the dorsal fin partially erect in a triangular shape to make the body outline more similar to that of the pufferfish. Many of the footballers change to the blue-spot colour pattern at around 20-25 cm TL when the mimicry is no longer effective or necessary but a number of individuals apparently do not bother to change colour until they are much larger, with occasional individuals reaching a length of 70 cm and still retaining the footballer colour pattern.

So we were actually dealing with three common species, the common coral trout, the bar-cheeked coral trout and the footballer/blue-spot coral trout, as well as the rare passionfruit coral trout.

Cross-Shelf Abundance

When the results from our surveys started to come in it became apparent that the major factor influencing where coral trout were found was the position of each reef on the gradient across the continental shelf.

Anyone who has driven up the Queensland coast will realise that coastal and inshore reefs are usually very turbid. Underwater visibility is usually poor and if you can see five metres you are lucky. When the trade winds are blowing the waves stir up the silt and turn the water brown, reducing visibility to less than a metre. Animals that live on coastal reefs must be able to cope with this turbid water and with the seaweed forests that are often found in shallow water on these reefs.

The bar-cheeked coral trout is able to thrive in these conditions, and is the only species of coral trout we found on the most turbid coastal reefs such as those around the Barnard Islands near Mourilyan Harbour. Average densities of bar-cheeked coral trout on these reefs was around eight per hectare (figure 2). On inshore reefs where conditions are a bit better, such as around the Palm Islands north of Townsville, on Low Isles off Port Douglas and on the Cape Tribulation reefs, this species was also common (10 per hectare) but a few common coral trout were encountered as well (about 7 per hectare).

On mid-shelf reefs that are less than half way across the continental shelf water conditions are usually a lot better, with visibility around 10 metres, although in windy conditions this may drop to about 5 metres. In these conditions the common coral trout starts to come into its own, with average densities of over 25 per hectare, but the bar-cheeked coral trout is still found on the back reef slopes here (about 5 per hectare). As we get out into the clearer water around the reefs more than half-way across the shelf where visibility in usually between 15 and 20 metres, we get into the heart of coral trout country; the combined numbers of all species of coral trout are about twice as high here as anywhere else (figure 2). The common coral trout is the only common species on these reefs with average densities of almost 60 per hectare. So imagine, if you will, next time you are on one of these reefs - the majority of reefs on the GBR fall into this category - and the fish are not biting that there are around 60 coral trout within a stones throw of your boat. We also start to find a few footballer/blue-spot coral trout on these reefs but only about 1.5 per hectare.

Figure 2.  Changes in numbers of the different coral trout species across the continental shelf from turbid coastal reefs to the front of outer shelf reefs washed by the clear waters of the Coral Sea.

When we start to get out wide, as the fishermen call it, out near the outer edge of the continental shelf where the underwater visibility approaches 30 metres, coral trout are less abundant. On the back of these outer shelf reefs the common coral trout is still the species most often encountered but only at densities of around 15 per hectare. The footballer/blue-spot coral trout becomes more abundant on these reefs (6.5 per hectare).

On the exposed front of these outer reefs, where the Coral Sea swell crashes unhindered, the reef slope is often very steep, falling quickly to depths of over 50 metres in visibility that often exceeds the same distance. Large blue-spot coral trout are dominant here, occurring at average densities of over 11 per hectare, and fish over 20 kilograms in weight are sometimes caught around these reefs. A few common coral trout are also seen on these steep front reefs but they do not seem to like these conditions and on reefs such as Raine Island they are not found at all.

When the footballer/blue-spot coral trout is found on mid-shelf reefs about half the individuals seen are of the footballer colour pattern and half are blue-spots. On the clearer outer shelf reefs the majority of individuals are of the blue-spot colour pattern; over four times as many as there are footballers. Such a result would be possible if footballers were changing colour to the blue-spot form at a larger size on mid-shelf reefs, or if blue-spots had far less chance of survival on these reefs. If either of these mechanisms were operating then the average length of the two colour forms would be different on mid-shelf reefs compared to outer shelf reefs. If we calculate averages for these fishes on the two groups of reefs from our estimates of length for all coral trout counted we find that they were very similar: 40 cm versus 39 cm for the footballer form and 58 cm versus 59 cm for the blue-spot form, implying that these mechanisms are probably not the correct explanation for the different colour pattern ratios. A more likely explanation is that blue-spot coral trout prefer conditions on outer shelf reefs and migrate between reefs until they reach an outer shelf reef, moving tens of kilometres across the open bottom.

We can sum up by saying that in general the three common species of coral trout replace each other as we move across the continental shelf, with the bar-cheeked coral trout found on inner shelf reefs, the common coral trout on mid-shelf reefs and the footballer/blue-spot coral trout on outer shelf reefs.

North-South Abundance

There were also some dramatic effects on coral trout numbers caused by the vast latitudinal spread covered by the GBR. The GBR reef complex stretches a distance of over 1,800 kilometres, from 10°30' south to 24° south, with a winter water temperature difference between the north and south extremes of almost 5°C. There are changes in reef type along this huge distance and we might expect there to be changes in the reef animal communities as well. To the north of Cape Tribulation there is an almost continuous rampart of outer barrier reefs along the edge of the shelf and reefs in this northern region are different from those in the middle of the GBR, between Cape Tribulation and Cape Upstart, where the reefs are smaller and more widely spaced, without the protection provided by the outer barrier rampart. To the south, reefs become larger and closer together again, but they are a long way offshore, and subjected to much stronger currents than elsewhere on the GBR. Very few reefs in this southern region are close to the outer edge of the continental shelf. At the southern end of the GBR, and separate from the main body of reefs, is the Capricorn-Bunker Group of about 22 reefs, that includes Heron Island and Lady Musgrave Island.

Figure 3.  Numbers of the common coral trout on mid-shelf reefs in four major regions along the length of the GBR.

The density of some of the coral trout species changes markedly along the length of the GBR, although these changes are not as dramatic as those that occur across the continental shelf. On those mid-shelf reefs where it is most abundant the common coral trout has very similar average densities throughout the northern and middle regions of the GBR. In the northern region there are an average of 28 common coral trout per hectare on mid-shelf reefs, while in the middle region the average is 35 per hectare (figure 3). As we get into the reefs of the southern region there are far more common coral trout with average densities of 76 fish per hectare. On some reefs in the Swain Group at the south end of this region we counted more than 150 coral trout in a hectare of transects. Diving on these reefs we often found ourselves surrounded by curious coral trout, with 20 or 30 fish is view at once. In the Capricorn-Bunker Group of reefs there were also a lot of common coral trout, with average numbers of around 50 per hectare.

We are not sure why there should be such large numbers of coral trout on the southern region reefs. From all the information available the levels of commercial fishing are higher if anything in this region compared to areas further north and there is a lot a recreational fishing from charter boats. It is possible that there is more food available to coral trout on these southern reefs. During the summer months large schools of hardyhead bait fish are found around these reefs but do not occur further north and diet studies have shown that these are extensively preyed upon by coral trout. It may be that the reefs can support higher numbers of coral trout because of the seasonal availability of this abundant food supply. Interestingly there were no differences in the numbers of this species along the length of the GBR on outer shelf reefs where the bait fish schools do not occur. There is probably insufficient plankton to support these schools of small fish in the clear oceanic waters around reefs near the edge of the shelf.

Figure 4.  Numbers of the footballer/blue-spot coral trout on outer shelf reefs in three major regions along the length of the GBR.

The footballer/blue-spot coral trout also shows marked latitudinal changes in density, but only on outer shelf reefs where this species is most abundant. For this coral trout there are higher numbers in the northern region, on the reefs of the outer barrier rampart, where there are an average of 9.5 fish per hectare, far more than the average of 3.6 per hectare on reefs further south (figure 4). In this case it is clearly some feature peculiar to these outer barrier reefs that these large coral trout prefer, but at this stage we have no real idea of what this could be. Numbers were even higher on the clear, steep fronts of these reefs with over 11 fish per hectare.

These surveys have given us some idea of the major patterns in the distribution of the different species of coral trout. If we want to say anything useful about the effect of fishing on these species we have to know something about the underlying natural density differences between reefs in different positions on the GBR. We documented order of magnitude density differences between groups of reefs across the continental shelf and over 2x differences along the latitudinal range of the GBR. We also got some information on the effects of fishing on coral trout density but that is a separate story.

© Copyright 2000 Sea Research
This document may be distributed freely if unmodified.