A technique for calculating bearing and tilt angles of walleye pollock photographed in trawls with digital still-picture loggers

Although visual information can help to monitor fish behavior in situ, quantitative analysis from images is difficult. There is little quantitative information from images on fish behavior in situ. In this study, we used digital still-picture loggers (DSLs) to monitor the bearing and tilt angles of walleye pollock, Theragra chalcogramma, in a trawl in situ. We attached two DSLs to a trawl net and recorded still pictures of fish near the mouth of the trawl. Eleven pictures of walleye pollock were obtained during 79 trawl surveys. From the still images, we calculated the bearing and tilt angles of the fish using the ratio of body length and body height. We confirmed the accuracy of the calculated angles by comparing the computed angles using image-processing software. The calculated angles agreed with the angles computed using the software. Therefore, our simple method can be used to monitor fish bearing and tilt angles in situ. Based on our results, bearing angles of walleye pollock were not shown in the same direction to a trawl. Furthermore, the fish tilt angles differed significantly between adults and juveniles.