By Alexander Tewfik, Ph.D and Myles Phillips; Wildlife Conservation Society, Belize Program
Remote sizing of fish using laser caliper technology is the most recent addition to ongoing data collection efforts conducted by Belize’s Spawning Aggregation (SPAG) working group. In 2016, the Wildlife Conservation Society piloted the use of this relatively simple innovation at the Northeast Point SPAG site on Glovers Reef.
The apparatus features two submersible lasers mounted under a handheld underwater camera system and is operated by a single diver. The operator places the two laser points on the lateral surface of a fish while recording video. During review of the video footage after the survey, data collectors can use the known distance between the two laser points on the fish’s body to extrapolate the actual size of the animal. The University of Belize – Environmental Research Institute has since deployed a similar system for use in their research at Maugre Caye, Turneffe Atoll reserve.
The apparatus is not expensive (roughly $200USD for lasers and mounting components), with the underwater camera representing the main cost. Video lights are not required; an advantage as the light causes distraction to the fish. It is not bulky or difficult to use in the hands of an experienced diver. The camera operator must position themselves perpendicular to as many fish as possible during their limited “bottom time” while maintaining their buoyancy, taking note of their surroundings, and remaining steady in the water for high quality video capture.
Although spawning aggregation monitoring has been limited to fish species and counts since 2000 (Burns-Perez and Tewfik 2016), fish sizing is important as one of the key indicators of sexual maturity and population health, and is directly used for management of fishing effort (i.e. size regulations). A healthy population of fish will include many individuals above minimum size at sexual maturity, indicating a healthy breeding population for perpetuation of the population and their associated fisheries. In addition, bigger fish produce exponentially more eggs (i.e. megaspawners) than their smaller, younger, conspecifics (Berkeley, et al., 2004). Spawning aggregations are comprised of breeding adults almost exclusively, and as such, the health of these groups is a direct representation of population viability.
The general methodology used for SPAG surveys follows the ‘Spawning Aggregation Monitoring Protocol for the Meso-American Reef and the Wider Caribbean Version 2.0’ (Heyman, et al. 2004), and the ‘Revised Sections of the Spawning Aggregation Monitoring Protocol (a supplement to the 2004 protocol that was prepared by members of the Belize Spawning Aggregation Working Group). Prior to the advent of laser sizing, size data was collected using the visual estimates of multiple divers; a common protocol known to be highly susceptible to surveyor bias. Surveyor bias occurs when the subjective judgment of each surveyor leads to an intrinsic error in measurement from each surveyor i.e. chronic overestimation or underestimation of fish size. Obviously, the most accurate way to measure individuals would be to capture and measure them out of the water; but laser sizing allows high accuracy and significantly lowered margin of error without interrupting the breeding activities of any individual fish. In addition, laser sizing is less time, effort and resource intensive than specimen capture, and causes minimal stress to the fish. This is especially favourable because spawning aggregation sites have exhibited trends of population decline or extinction since the late 90s, and the trauma of capture can lead to injury, reduced reproductive success, and increased susceptibility to predators and disease (Heppell, et al., 2012).
The use of this apparatus frees other divers to perform other vital monitoring tasks, as only the camera operator needs to spend time collecting size information. The effective range of the lasers used by WCS was 5-10m on larger fish in clear water, meaning that camera operators need to get close, but not too close to the breeding animals to take useful footage. The video can be reviewed as many times as necessary, and can collect associated information passively e.g. the presence and density of associated species including egg predators and carnivores.
The use of laser calipers is highly effective for collecting fish size data under the depth, light and current conditions typical of grouper and snapper spawning sites, and have been used to confirm the presence of large and mature fish at multiple sites within Belize’s reef systems. The SPAG working group intends to promote the use of this apparatus by all its partners in order to collect the most useful data to inform management of these endangered species and their breeding populations.
