Goliath East West

[In Libman]

Goliathgrouper have a pelagic larval duration of approximately 6 weeks. They settle in shallow mangrove habitat, first in mangrove leaf litter, and then along mangrove shorelines. This habitat is absolutely essential to their survival and ability to recruit to adult populations. The juvenile stage lasts 5 or 6 years in this mangrove habitat, after which fish egress to shallow reefs, eventually joining adult populations offshore. Their distribution in mangroves depends on local water quality, particularly dissolved oxygen content (> 4 ppm), mid-range salinities (> 10 ppt), and warm water (> 51F = 11C). This species is vulnerable to low water temperatures which have been known to wipe out entire year classes when falling below 51F.

This species occurred historically in tropical and subtropical waters of both the western Atlantic (both coasts of Florida, the Gulf of Mexico, the Caribbean south through the coasts of Brazil) and the Eastern Atlantic off the coast of tropical East Africa. Populations are on decline throughout their range and have all but vanished from East African waters.

Low-relief natural reefs, high-relief artificial reefs, and shipwrecks that dot the east and west Florida shelves, provide the architectural complexity that Goliath Grouper and many other reef fishes seek. Goliath Grouper enhance the structural complexity of these sites by excavating sediment from around the reef base, and in so doing, increase the abundance and diversity of other resident species. Most adult Goliath Grouper are found either alone or in small groups in relatively shallow water on these reefs and wrecks outside of the spawning season. They migrate during the summer often up to 100 km from inshore to spawning sites occurring at depths up to 50 m. Here, they aggregate in groups of roughly 100 individuals typically on rock ledges, isolated patch reefs, or ship wrecks. Spawning occurs from July through September in the northern hemisphere and from December through February in the southern hemisphere.

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We present data on the distribution of goliath catfish (Brachyplatystoma) adults, larvae and juveniles across the Amazon Basin, including areas in or near the Andes. Even without tagging experiments, the general differential distribution of sub-adult (downstream) and adult (upstream) goliath catfish size-classes9, as well as otolith microchemistry data10, strongly indicates that long-distance upstream goliath catfish migration occurs. Seasonal upstream goliath catfish movements are also visually obvious at some cataracts, such as those of the Madeira River (Fig. 1) in Brazil, before dams were built4, and those of the Caquet River in Colombia6. Therefore, it is reasonable to hypothesize that long-distance downstream migrations of young fish occurs, otherwise there would be no recruitment to nurseries, in some cases as far downstream as the Amazon River estuary.

Fish length and gonadal data derived from specimens captured in fisheries in major Andes-Amazon sub-basins in Brazil, Bolivia and Peru, complemented by published data from studies based in Colombia19,23 and Ecuador24, show the wide distribution of mature B. rousseauxii, B. platynemum and B. juruense in all major turbid rivers with headwaters in the Andes, including the Amazon River main channel (Fig. 2 and Table 1). Mature fish are defined as ripe individuals, that is, individuals with fully developed ova or testes25. Brachyplatystoma rousseauxii has the widest distribution. In addition to whitewater (turbid) rivers arising in the Andes, it is found in many clearwater and blackwater tributaries that arise on the Brazilian and Guiana Shields. With the exception of the lower Tocantins River26, which is part of the Amazon River estuary, B. rousseauxii is rarely registered in the fisheries of blackwater and clearwater rivers, an indication of its rarity in these drainages. The only Brazilian or Guiana shield rivers where mature long-distance migratory goliath catfishes were found was the Branco, a semi-turbid tributary of the Negro River. Of the goliath species considered, only B. rousseauxii was present in the Branco River, but it is of minimal importance in fisheries there and is reported by fishermen to be relatively rare.

Monthly precipitation and monthly capture of mature B. rousseauxii, B. platynemum, and B. juruense between April 2002 and April 2004 in Puerto Maldonado (Madre de Dios River, Upper Madeira Basin) and between July 2004 and July 2005 at Atalaya (near the confluence of the Ucayali and Urubamba Rivers). B. juruense was not captured in the Madre de Dios River during our study period. Precipitation was used as a proxy for river level since there were few data for the latter in the Andean region.

All 5,348 B. rousseauxii, 2,985 B. platynemum and 528 B. juruense specimens we examined from commercial fisheries near the Andean Piedmont or in the immediate pre-Andean area were sexually mature adults with fully developed gonads or recently spawned fish (Table 2). In contrast to the downstream regions, all goliath catfish captured near the Andean Piedmont had empty stomachs, suggesting that their presence in these areas was not related to trophic migration but to spawning. Although goliath catfishes were present in every month in the far western Amazon, the commercial captures from which our data were derived indicated strong seasonal variation (Fig. 3). The seasonal differences between commercial captures in the Madre de Dios (upper Madeira Basin) and Ucayali and Urubamba Rivers (Ucayali Basin) (Fig. 3) may reflect distinct headwater migration patterns or differences in fishing, considering that fishing was virtually halted in the Madre de Dios during the high water discharge period (November-February) due to the large quantities of downstream-moving wood that are a danger to nets.

In the Ucayali and Urubamba Rivers, ready-to-spawn B. rousseauxii, B. juruense and B. platynemum were most abundant in commercial fisheries during the rainy and warmer months corresponding to the higher river discharge period from October to March (Fig. 3). Since monthly fishing effort was approximately equivalent, landings can be considered a proxy for seasonal migratory fish abundance. Furthermore, goliath catfishes were captured by commercial fisheries in river channels with downriver drifting gill nets during all months. Since drifting gill nets only capture fish moving upstream, they reveal the direction of movement.

The monthly drifting larvae densities of goliath catfishes in the Madre de Dios River of the upper Madeira Basin in relation to total monthly precipitation (mm) levels. Larvae densities were estimated based on ichthyoplankton samples from the Madre de Dios River between November 2004 and August 2005. Precipitation data were used as a proxy for river level since data for the latter were not available. Precipitation data are for the city of Puerto Maldonado were supplied by the Servicio Nacional de Meteorologa e Hidrologa del Peru (SENAMHI).

A seasonal factor for all species is that varying current speeds during sampling periods could affect the larval size distribution along the downstream movement. If current speed greatly influenced the distribution of juvenile size, then a much less precise pattern would be expected for B. rousseauxii since it spawns during varying periods of the year with different current speeds. However, B. rousseauxii shows the best fit of all species to the downstream migratory growth model.

The wide distribution of small larvae of B. platynemum in the Madeira Basin and the Amazon as a whole and their highly mixed length-class distribution do not fit exponential curves, strongly suggesting that spawning for this species is widespread, that more than one population exists33 and that the nurseries include a large area in the western and central, and perhaps even eastern, Amazon (Figs 5 and 6).

The presence and abundance of mature goliath catfish in commercial fisheries in the Andean region is a reliable indicator of upstream movement to spawn. The absence of these fish in commercial fisheries during some or all months near the Andes, however, should not be interpreted as direct evidence that they are not present in the region, as fishing activity must also be considered. The striking differences between the monthly relative captures in the Madre de Dios and Ucayali-Urubamba Rivers is most likely due to the absence of fishing in the former rather than the absence of mature fish. The high catches in the Madre de Dios River during the falling river level period during all three years for which data are available should not be interpreted as a greater abundance at this time of year but rather that fishing is possible during these months (Fig. 3). The presence of spawning B. rousseauxii and B. platynemum in the Madre de Dios River channel during the high water period is corroborated by the relative abundance of their larvae (Fig. 4). The higher density of fish larvae during the rising water period is thus a better indicator of the main reproductive period of goliath catfishes than monthly commercial captures. The best indicator, however, is larval flux, which is the absolute density value of larvae per unit of time in a river section38. Drift densities of young fish generally decrease with increased river depth and flow velocity due to the dilution effect caused by higher turbulence in a much greater volume of water39. The larval flux index was not used due to the difficulty of obtaining accurate river discharge data near the Andes because of the few hydrological stations that exist. Considering that goliath catfish larvae densities were calculated without discharge calculations and were highest at the beginning of the rainy season and decreased during the January and March peak discharge periods30, it is possible that the highest larval flux would be in the latter period if river discharge was considered in the larvae density algorithms.

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