ENVIRONMENTAL AND SOCIAL IMPACT OF SUSTAINABLE OFFSHORE CAGE CULTURE PRODUCTION IN PUERTO RICAN WATERS

Preliminary results

Water quality analysis

Water samples were taken at the bottom, mid, and surface depths. We also took samples at three depths in a control site (Co) (about 400 m from the cages) to determine if the changes in values are due to the effect of the cages or seasonality. Water samples were taken at north (N), south (S), west (W) and east (E) of each cage, and at 20 and 40 m, and at the center of the cage.

Water analyses indicated no increases in ammonia-N in the bottom samples for the snapper and cobia cages (Fig. 1-2). October and December had similar concentrations of this nutrient at the bottom. Although December shows higher concentration of ammonia-N at mid-and surface waters, they appear due to a seasonality effect because the control site had similar concentrations (Figs. 3-6). It is noted that the concentrations for all months in both cages have been relatively low and normal for this waters (< 3.5 µmol/L).

Nitrite concentrations were low for all the months analyzed (< 3.0 µmol/L) (Figs. 7-12). Nitrite concentrations were higher in August (when there was no nutrient input since the cages were stocked on that date).

Nitrate concentrations were also low for all months analyzed (< 2.5 µmol/L) (Figs. 13-18). Nitrate concentrations were higher in December at the bottom depth and in August at the surface waters apparently due to the seasonality changes because the control site had similar concentrations.

Phosphate concentrations were also low for all months analyzed (< 1.0 µmol/L) (Figs. 19-24). The controls site had similar concentrations suggesting that changes are seasonal.

Dissolved oxygen concentrations during December and January were 88.7% of saturation, with an average concentration of 5.8 mg/L (Fig. 25).

The water temperature declined slightly from December to January (Fig. 26), with an average of 27.4şC. It is noted that the coldest months for Puerto Rican waters usually are December, January, and February (the coldest month).

The chlorophyll-concentrations around the cages during December and January had an average concentration of 10.1 µg/L. It increased slightly during December, but has remained similar from January (Fig. 27).

The water turbidity remained below 20 NTU during December and part of January (Fig. 28). However, the last five days before the Hydro lab recovery, the turbidity increased significantly, reaching 100-120 NTU at January 15-16, apparently associated with the net cleaning process. Please note that Snapperfarm, Inc. cleaned the nets during these days. Thus the increase in turbidity was a temporary situation.

Macroinvertebrate benthic fauna associated with the cages

The total number of micro-benthic organisms near the cages (40 m) during October was almost the double the organisms at the control site. During December, the total number of organisms increased to more than three times the number of organisms found at the control site (Fig. 29-30).  These preliminary results suggest that the nutrient input around the cages has resulted in an increase in the macroinvertebrate population. The highest numbers were found near the cobia cages, the cage receiving the most feed. However, initial indications appear that the biodiversity of these organisms has not changed; relative compositions between the snapper and cobia cage are similar (Fig. 31-32).

Biofouling in the cage net

This study examined the fouling organisms on the nets of two offshore fish culture cages located on the southern coast of Culebra, Puerto Rico.  Both the diversity and percentage coverage of fouling organisms tended to be less on the samples below the cages’ rim.  On both cages, the samples above the rim were mostly covered by algae, but bryozoans and hydroids were also found.  Samples below the cage rim were dominated by hydroids on both cages with some ascidians. On the southern side, the snapper cage had a higher coverage percentage (65.2%) on the shallow sample (i.e., above the rim), whereas the cobia cage had a higher percentage (32.7%) on the deeper sample. Meanwhile, on the northern side, the cobia cage had higher percent coverage on the shallow sample (58.1%), whereas the snapper cage had a higher percent coverage.  In terms of diversity, the snapper cage showed a greater amount of diversity of sessile organisms, while on the cobia cage the diversity of mobile organisms was greater.

Icthyofauna around the cages

Videos taken before the cages were installed indicated few fish in the water column. Videos taken from June 2002 to January 2003 suggest that there are significant increases in the numbers of fish found near the cages. This study gives a qualitative description of the composition, distribution and relative abundance of fishes associated with offshore-submerged cage systems for the culture of snapper in Puerto Rican waters. Censuses were taken at two submerged cages in Culebra Island, Puerto Rico. Each cage was sample by recording species and relative abundance of fishes at different depths (top, ring and bottom of the cage). It is noted that the information presented here is from 4 of 6 videos taken. A total of 22 species belonging to 12 families were recorded beneath cages. By far the most abundant observed family was Carangidae with four species, (Caranx rubber, Caranx crysos and Decapterus sp),. but also Haemulidae, Acanthuridae and Labridae presented high abundances. The genus Caranx had been previously considered to have species with a tendency to associate with floating objects (Hunter et al. 1967). Jacks are considered strong swimming predators of open sea with a clear tendency of forming large schools (Humann 1994).  Some species were present in large schools and some others were also present in small quantities or solitary. Differences in juvenile’s distribution in depth were found. These cages may have important ecological consequences since they act like nursery structures for some species, having a possible effect on the redistribution of juveniles. Six of twelve species were present with juvenile organisms. These were: Aulostomus maculatus, Mycteroperca venenosa, Lutjanus mahogony, Holacanthus ciliaris, Acanthurus coeruleus and Acanthurus chirurgus. The genera Acanthurus was present only in juvenile stages and its distribution was limited to the top of the cages. Thalassoma bifasciatum was limited to the top of the cage too. None other specie showed difference in depth distribution.Most of the species encircling the cages moved in schools, some of them in large ones and others in less dense ones depending on the specie. Decapterus sp. presented the bigger and compacter schools, possibly because of the smaller sized individuals. Deudero et al. (1999) affirm that the size of a shoal is quite variable depending on the ecology of the species and on its propensity to be preyed upon. They reported carangid species making use of the schooling tendency, as either a defense against predators or as a mechanism for an enhanced predators foraging activity. The location of the cages may have an effect on wild fish distributions. All these species censed are present on coral reefs, but some of them are more common on open waters, like jacks, Sphyraena barracuda or Scomberomurus cavalla. The submerged and floating objects represent a substrate for many organisms (Hunter et al. 1967) and there begins the aggregation effect of the cages. One explanation is the attraction to drifting materials as schooling companions; the other is the fact that they provide a substitute substrate for species undergoing a change to another mode of existence (Hunter et al. 1967). It is believed too that submerged objects should give rise to an increase in complexity of the media (Deudero et al. 1999) that increase this attracting effect. Since these fishes are opportunistic feeders (Humann 1994) its presence around the cages could be attributable to food searching. The presence of species exclusively associated to coral reefs like Holacanthus ciliaris can be explained by the fact that Culebra is an island surrounded by coral reefs, and although the cages are located far enough from these coral areas they can freely navigate across discontinuities within their home range, helped by the strong currents of the zone (Appeldoorn et al. 1997).

In the data analyzed, the presence of species with juvenile organisms have been high (approximately 26%, 6 species). Many reef fishes use a variety of habitats during ontogeny, often showing distinct spatial separation in settlement areas, nursery areas, juvenile and adult feeding areas (Appeldoorn et al. 1997). All the observed specimens of the genus Acanthurus were juveniles and were mainly distributed in the top of the cage. The fact that there is a considerable aggregation of recruits in an area means that either reproduction or survival is patchy (Hall and Lennert. 1992). In this sense, it is important to note the importance of these cages possibly acting as a nursery structure where juveniles could find the conditions required for existence and survival. The same results have been reported by other studies of floating objects (Deudero et al. 1999). These cages may have important ecological consequences since they act like nursery structures for some species, possibly having an effect on the redistribution of juveniles.

The species that exhibited a solitary behavior were the bigger ones, like Sphyraena barracuda, Echeneis naucrates and Scomberomus cavalla, among others. These species seems to take advantage of their large body size and represent the higher level on the trophic level around the cages.The lack of studies carried out on similar habitats does not allow close comparisons. However comparing with FAD’s studies the richness value obtained here is rather high, taking into account that 22 species belonging to 12 families were found. Since the results reported here correspond only to the fish community associated to the submerged cages between three months, this suggests that the aggregation effect of the cages is really strong to the wild fish community.

Weather information

The average tide, air temperature, heat index, dew point, wind speed,  and relative humidity are shown in Figs. 32-36. (not yet available on this site)

Miscellaneous

Lobster populations range from 150 to 250 outside of the cages. They appear to be increasing in size. At least one cultured fish had lobster antennae protruding from their mouths. Fishes have been observed eating waste or possibly uneaten feed from the cages.

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