Project Report PR-94-12
An Analysis of the Fish and Macrobenthos Along the Sand Island Deep Ocean Outfall, Oahu, Hawaii, Using Remote Video IV, 1993
Richard E. Brock
Because the diffuser of the Sand Island deep ocean outfall lies below safe diving depths, a remotely controlled video camera system was used to determine the status of the fish and diurnally exposed macrobenthos resident to the diffuser. The use of a remotely operated vehicle is stipulated in the National Pollutant Discharge Elimination System 301(h) waiver permit for the Sand Island Wastewater Treatment Plant. Video reconnaissance was completed over the entire 1 036 m length of the outfall diffuser. Five visual “transects,” which “sampled” approximately 41% of the total diffuser length, were established on the diffuser pipe. Video sampling of the diffuser marine communities was carried out in 1990, 1991, 1992, and 1993. Only a few species of diurnally exposed macroinvertebrates are evident on the videotapes of the diffuser; the numbers are insufficient for any meaningful analysis. In 1993, 22 fish species (279 individuals) having an estimated biomass ranging from 6 to 39 g/m2 (mean 21 g/m2) were censused; in 1991, 27 species (1,785 individuals) having a biomass ranging from 8 to 106 g/m2 (mean 42 g/m2) were counted; and in 1992, 30 fish species (2,936 individuals) having an estimated biomass ranging from 39 to 77 g/m2 (mean 53 g/m2) were censused. Because the 1990 video census covered only the terminal 183 m of the diffuser, whereas the later surveys were spread out along the entire diffuser length, a direct comparison cannot be made between the 1990 Data and the Data for subsequent years. In 1990, one “new” fish species was encountered for every 22.9 m2 of substratum sampled and one fish was seen for every 5.6 m2; in 1991, it was one new species for every 13.1 m2 and one fish for every 0.7 m2; and in 1992, it was one new species for every 7.4 m2 and one fish for every 0.4 m2. The 1993 census noted one new fish species for every 38.5 m2 of substratum sampled and one fish for every 3.0 m2. In the 1991 – 93 period, measures of the fish community (number of species, number of individuals, and biomass) increased from 1991 to 1992 but decreased in 1993. From a statistical perspective, the change in the mean number of species per transect and the mean number of individual fishes per transect is significant (Kruskal-Wallis ANOVA); changes in the biomass of fishes over this time are not significant. These changes in the fish community are attributed to changes in resolution of the videotape from which the Data are derived. Poorer camera resolution results in lower counts; camera resolution is affected by local wind and currents interacting with the camera, tether, and support vessel as well as by water visibility. Controlling these sources of variation inherent with the use of the remotely operated video system is difficult if not impossible. Until an alternative can be found, the remotely controlled video system is the only low-cost means available to view the marine communities on the diffuser. Until a more accurate means of visual assessment is available, the biological Data generated by the remotely operated video camera should be viewed as qualitative, with little statistical rigor.