Snappers are economically important species for fisheries worldwide. To determine the health of snapper populations, we must monitor their abundance and record any changes in their biomass. NOAA scientists conducted a study to test new, more efficient and accurate methods to regularly monitor snapper in the Hawaii bottomfish fishing grounds. The results of this study were recently published in the ICES Journal of Marine Science.
Snappers are abundant in nearshore, 100–400-meter deep subtropical and tropical oceans with rocky, uneven bottoms that have steep slopes. They typically occupy areas near the ocean floor but are also found further in the water column and within crevices or under outcrops.
Modern research methods limit accurate and efficient abundance estimations for these species. Trawl sampling is not feasible as the highly uneven, rocky bottom would damage the net and the fragile habitat itself could be disturbed. More recent underwater video observations are limited by light and field-of-view distance. Placing video cameras in the water can also scare away or attract fish, skewing the results.
Using Active Acoustics to Count Fish
Scientists have begun using active acoustic methods to estimate fish abundance. Active acoustics emit a pulse of sound, which echoes off objects to produce a signal that can be analyzed for certain characteristics—in this case, fish. Active acoustics are non-intrusive and not limited by light. They rapidly and efficiently collect data over the water column.
Different fish species produce different signals depending on their size, orientation, gas-bladder characteristics, and physiology. A variety of fish with different characteristics can produce similar signals. Snappers mix with other species, so it is particularly challenging to separate acoustic signals from other species.
Researchers at the Pacific Islands Fisheries Science Center conducted a study in the Hawaiian Archipelago. They wanted to determine how accurately we could detect snapper species and estimate their abundance and biomass with active acoustic methods. Using both acoustics and video observations, they paired specific echoes with fish of known species and sizes. They identified particular characteristics to determine whether an echo was returned from a snapper or another species. “For these snappers, the important echo characteristics are echo strength, individual swimming pattern, depth, distance from bottom, group density, number of fish in loose groups, and shape of tight groups of fish,” says Dr. Domokos.
These acoustic characteristics allowed our researchers to separate Deep-7 echoes from those of other fish. The exception was loosely grouped jacks at the large end of Deep-7 size range, locally known as kahala, plus four tightly-grouped smaller fish species. “If we had additional data, we could successfully separate at least three of these fish types, kahala, unicornfish, and mackerel scad,” predicts Dr. Domokos. If the acoustics methods prove unsuccessful for the other two species, we can eliminate their biomass from the total count by simple thresholding. They occupy the shallowest depths always near the bottom, and have the smallest sizes of Deep-7.
Once we finesse the methods, active acoustics can be adapted to monitor snappers worldwide. However, we can even use the current acoustic characteristics to determine relative biomass and monitor over time to assess changes in the amount of snappers out there. We can use this valuable information to inform the fishing practices used to collect snappers and ensure the fisheries are sustainable.