Study on improving jigging capture techniques for neon flying squid, Ommastrephes bartramii, in the North Pacific Ocean

The aim of this study is to improve jigging capture techniques for the neon flying squid, Ommastrephes bartramii, and identify the mechanism of, and countermeasures thereto, of frequently-observed catch loss due to hooked squid falling off. In Chapter I, the history of neon flying squid fishing is summarized, problems of current squid fishing are mentioned, and a direction toward a technological solution is indicated. This chapter reports that drift net fishing of neon flying squid started in the 1970s, but that in 1992, a United Nations moratorium on the use of drift nets in the open seas resulted in a change in 1993 in neon flying squid fishing from drift net use to jigging (line catching). Today, most of the fishing boats operating in the area are Japanese, Taiwanese or Chinese. Chapter II describes the finding, obtained by DNA analysis, that the neon flying squid inhabiting the fishing grounds in the North Pacific Ocean belong to a single genetic group and are a resource requiring international fishery management to prevent overfishing by a growing number of fishing boats. For appropriate fisheries management of the neon flying squid in this fishing ground, the actual conditions of the jigging capture process need to be understood. The results of surveys in 2003 to 2008 are summarized in the subsequent five chapters. Chapter III investigates the mechanism of tentacle breakage during jigging as a possible cause of the fall-off event. Chapter IV describes the relationship between boat motion and falling-off of squid, and in Chapter V, transmission of boat motion to fishing lines is elucidated, providing a pointer to understanding the event of hooked squid falling off. To reduce squid fall-off from the jig, the effects of increased hauling speed of fishing line on squid hooking are analyzed in Chapter VI. Chapter VII describes a thorough examination of the hauling speed of jigging machines, aiming to reduce the fall-off ratio and increase the catch. In Chapter VIII, improvements of techniques for fishing neon flying squid and fishery management methods are discussed after integrating the results. From an observation of upward reeling of jigs, the fall-off event of squid was classified into the following three patterns. Cases where only tentacles remained on the jigs are named 'underwater fall-offs.' Fall-offs after the squid have come out of the water but before touching the guide roller are 'above-water fall-offs,' and those that fall off while in contact with the guide roller are called 'fall-offs upon contact.' According to a survey of fallen-off squid conducted over three years from 2006 to 2008, the total fall-off ratio accounted for 36% of all squid hooked. The ratio of underwater fall-offs was notably high. The fall-off ratio of squid that had hooked by only one or more tentacles accounted for 92% of all fallen-off squid. When the relationship between mantle length and fall-off ratio was analyzed using a logistic curve, the fall-off ratio was found to be high in small squid: the mantle length (ML) at which 50% (Fsub(50)) fell off from the jig was estimated to be 37.4 cm. Observation of hooked conditions of caught and fallen-off squid and their body size showed that caught squid that had hooked by three or more arms were significantly larger than those that were hooked only by one or more tentacles. The high fall-off ratio of squid smaller than Fsub(50) was therefore likely attributable to the squid capturing the jig with one or more tentacles, followed by breakage of the tentacles. Squid larger than Fsub(50) have greater propulsive power, and are thus more likely than smaller squid to embrace the jig with their arms rather than their tentacles, resulting in a lower fall-off ratio. Measurement of the breaking strength of a tentacle of a squid smaller than ML41.4 cm showed that the tentacle broke under a force close to its body weight. This suggests that small squid are prone to tentacle breakage if they capture the jig using only one tentacle. Past surveys have reported high fall-off ratio from jigging machines installed at either the bow or stern during stormy weather. In this study, the author investigated the reproducibility of the relationship between fall-off ratio and boat motion, and conducted a logistic regression analysis to investigate the effects of boat motion on squid fall-offs. A survey was conducted on a squid jigging vessel, the Hakurei Maru No.8 (276 GT, automatic squid jigging machines: 21 on the port side and 22 on the starboard side), in 2003 to 2005. The numbers of squid captured and those that fell off per jigging machine were counted once daily over 2 hours around sunset or after dark. When the sea was calm, with a wind at Force 4 or below, fewer squid fell from jigging machines that were nearer to the bow or stern and which had a longer cradle attached. In other words, a pattern of significantly lower fall-offs was observed for fishing lines that were relatively strongly influenced by pitching and rolling when the sea was calm. This was likely attributable to the capturing behaviors of this species. The motions of the jig caused by boat motion appeared to have triggered the squid to grasp the jig with both its arms plus its tentacles. On the other hand, with a wind at Force 5 or more in stormy weather, the fall-off ratio was significantly higher in machines at the stern and bow. To analyze this event, the author investigated whether or not the boat's motions were transmitted to the jig by measuring the motions of both the vessel and jigs using an M380L-D2GT acceleration data logger (Little Leonard Co. Ltd., Tokyo, Japan, hereinafter 'acceleration logger'). The power spectral density (PSD) was calculated using fast Fourier transform (FFT) to investigate the properties of acceleration signals recorded at the guide roller and fishing line. The calculation revealed that the spectrum detected at underwater jigs had the same period as the dominant period (0.125-0.090 Hz) of the boat motion detected at the guide roller, showing that boat motion was in fact transmitted to underwater jigs. During stormy weather, large motions of the boat are transmitted to the jigs, and this appears to cause tentacle breakage before the squid is able to embrace the jig with its arms. To reduce fall-offs of squid from a jig, we need to understand the relationship between the motions of a jig while it is being hauled upwards and the hooking state of neon flying squid. The jigs' motions were measured by installing an acceleration logger on a fishing line used for squid jigging, where the wire and nylon gut joined. While detecting sudden increases in load exceeding 6 kgf at the jigging machine, which were displayed on the integrated control and display, the time and depth of capture were recorded. The behaviors of a neon flying squid while it captured a jig were then assessed from the data and the accelerogram of the acceleration logger. The upward speed of the fishing line one second before a neon flying squid captured a jig was calculated using the difference in depth between two adjacent depth data, which were also recorded during the survey. The catch or fall-off of the said squid was also recorded. We noted that a squid embraced a jig with its arms when the upward speed of the fishing line was around 1.5 m/s but captured the jig with only its tentacles when the speed was about 2.5 m/s. This observation suggests that there is a threshold value of upward speed of a fishing line that determines whether a neon flying squid catches a jig with its tentacles or embraces it with its arms. To measure relative fall-offs of neon flying squid during jigging, the drum hauling speed was altered. Six different drum hauling speeds, ranging from 50 to 80 revolutions per minute, were tested and their relationship with the fall-off ratio was investigated. A drum hauling speed of 50 rpm was found to result in a low fall-off ratio. To estimate the upward speed of the fishing line at which a neon flying squid is most likely to capture a jig with its tentacles, the relationship between the tentacle-hooked ratio (the ratio of tentacle hooking as a proportion of all hooking) and upward jig speed was investigated. The upward jig speed at which a half of all squid capturing a jig caught the jig only with one or more tentacles was designated the 50% tentacle-hooked ratio (Rsub(50)) and was inferred to be 1.75 m/s. It was deduced that the tentacle-hooked ratio increases at upward jig speeds that exceed Rsub(50), resulting in a high tentacle-hooking ratio and fall-off due to tentacle breakage. In respect to the relationship between drum hauling speed and Rsub(50), 60 rpm or slower resulted in an upward jig speed within the 0.92-1.65 m/s range, or slower than Rsub(50). On the other hand, when the drum hauling speed was 65 rpm or faster, the upward jig speed rose to 1.79-2.20 m/s, and speeds greater than Rsub(50) were detected. A demonstration experiment was conducted based on these results. When squid were fished by sinking the tip of the line to a depth of 150 m, the number of squid caught per hour per line (CPUE) was the greatest at 70 to 75 rpm. This was because the fast drum hauling speed increased the number of squid caught per stroke or cycle of drawing and pulling the line. Lowering of the drum hauling speed to 50 rpm did not increase the CPUE in this experiment, likely because large squid of Fsub(50) or larger accounted for 64% of all squid. The catch can therefore be increased by increasing the drum hauling speed to 70 to 75 rpm if the squid are ML35 cm or larger and reducing the drum hauling speed to reduce the fall-off ratio if the squid are smaller than ML 35 cm. This study revealed that the mechanism of neon flying squid falling off from a jig is due to the squid capturing a jig with one or more tentacles followed by breakage of the tentacle during hauling. The author proposes a possible means of reducing fall-off by controlling the drum hauling speed and the rational use of resources by searching fishing grounds for larger squid.