Pelagic longline gear depth and shoaling

Abstract

Temperature-depth recorders (TDRs) were attached to pelagic longline gear in the Hawaii-based commercial fishery to obtain actual fishing depths and to test the accuracy of catenary algorithms for predicting fishing depths. Swordfish gear was set shallow by typically deploying four hooks between successive floats. The observed depth of the settled deepest hook had a median value of 60 m for 333 swordfish sets. Tuna longline gear deployed more hooks between floats (mean = 26.8), and the observed median depth of the deepest hook was 248 m (n = 266 sets). Maximum gear depth was predicted from estimates of the longline sag ratio and catenary algorithms; however, depth was not predicted for all TDR-monitored sets because estimating sag ratios proved problematic. Swordfish sets had less slack in the main line and correspondingly smaller catenary angles (median = 54.2• ) than tuna sets (median = 63.7• ). Median values of the predicted catenary depth were 123 m for swordfish sets (n = 203) and 307 m for tuna sets (n = 198). Shallow swordfish sets reached only ∼50% of their predicted depth, while deeper tuna sets reached about 70%. These values indicated that capture depths using traditional catenary equations may be biased without the benefit of TDRs affixed to longlines. Generalized linear models (GLMs) and generalized additive models (GAMs) were developed to explain the percentage of longline shoaling as a function of predicted catenary depth and environmental effects of wind stress, surface current velocity, and current shear. The GAM explained 67.2% of the deviance in shoaling for tuna sets and 41.3% for swordfish sets. Predicted catenary depth was always the initial variable included in the stepwise process, and the inclusion of environmental information in the GAM approach explained an additional 10-17% of the deviance compared to the GLMs. The explanatory ability of the environmental data may have been limited by the scale of the observations (1 • in space; weekly or monthly in time) or the geometric (transverse versus in-line) forcing between the environment and longline set. Longline gear models with environmental forcing affecting shoaling may be improved in future studies by incorporating contemporaneous environmental information, although this may restrict analyses to fine-scale experimental longlines. Published by Elsevier B.V.

Spatio-temporal distribution and target species of longline fisheries off Southeastern/Southern Brazil between 2000 and 2011

“…However, a greater variation of this time (3-15 hours), certainly related to the time taken by the search for albacores, is motivated by the growing trade in fresh tuna. BIGELOW et al (2006) in a study conducted in Hawaii also showed that the soak time is increased when the vessels attempt to capture swordfish (between 11 and 28 hours), and albacores (between 13 and 31 hours). A study of the longline fleet based on São Paulo state, found a mean soak time of 20 hours (MOURATO et al, 2011), which is much higher than that recorded in this study.…”

Comparative analysis of depth distribution for seventeen large pelagic fish species captured in a longline fishery in the central-eastern Pacific Ocean

“…where k is the sagging ratio defined as the length of horizontal line divided by the length of mainline between unit baskets and estimated as the ratio of the speed of the mainline thrower to speed of vessel (Bigelow et al 2006). In this study, speed of the line thrower and speed of vessel varied slightly during the trip.…”

“…The depth at which fish are captured can provide critical information for understanding the impacts of longline fisheries on targeted and bycatch species (Bigelow et al 2006). Deploying longline hooks at appropriate depths can greatly improve catch of desired species, such as bigeye tuna (Thunnus obesus; Suzuki et al 1977, Boggs 1992) and billfishes (Boggs 1992), and reduce bycatch of protected species and untargeted species, such as sea turtles (Gilman et al 2006, Beverly et al 2009).…”

“…acoustic, archival, and satellite) equipped with pressure sensors are an appropriate tool for observing vertical movements of pelagic fish in their habitat (Bach et al 2003, Musyl et al 2003, Schaefer et al 2009, Walli et al 2009, Stevens et al 2010. Longlines equipped with time-depth recorders (TDRs) and hook timers provide information on the time and depth of capture for many pelagic species (Boggs 1992, Bach et al 2003, Bigelow et al 2006. One advantage of TDRs and hook timers, compared with electronic tagging studies, is that a large number of individuals of different sizes and species in different environmental conditions can be sampled (Bach et al 2003).…”

“…Therefore, the catenary method has been frequently used for estimating hook depth in pelagic longline fisheries by using empirically derived correction factors (e.g. Hinton and Nakano 1996, Yano and Abe 1998, Bigelow et al 2006, Ward and Myers 2006.…”