189 National Marine Fisheries Service Fishery Bulletin First U.S. Commissioner established in 1881 of Fisheries and founder NOAA of Fishery Bulletin Abstract—A recent study determined Efficacy of 2 common bait types in reducing that when simultaneously exposed to 2 different commonly used baits, certain bycatch of coastal sharks on bottom longline shark species demonstrate preferences for a specific bait on bottom longlines. gear in the absence of choice To further investigate the value of bait type to reduce shark bycatch, we con- William B. Driggers III (contact author)1 ducted single- bait- type bottom longline Kristin M. Hannan2 sets with standardized gear baited with either mackerel or squid. For 4 of Email address for contact author: [email protected] the 5 shark species captured, there was no significant difference in catch rates 1 with bait type. However, catch rates of Mississippi Laboratories Atlantic sharpnose sharks (Rhizopri- Southeast Fisheries Science Center onodon terraenovae) were significantly National Marine Fisheries Service, NOAA higher on mackerel- baited hooks. Our 3209 Frederic Street results indicate that the use of squid Pascagoula, Mississippi 39567-4112 as bait can reduce the catch of at least 2 Riverside Technology Inc. one shark species in the northern Gulf Mississippi Laboratories of Mexico while not reducing the catch Southeast Fisheries Science Center of a targeted species, in this case, the National Marine Fisheries Service, NOAA red snapper (Lutjanus campechanus). 3209 Frederic Street However, because some protected spe- Pascagoula, Mississippi 39567-4112 cies, most notably sea turtles, have been shown to have higher catch rates on squid- baited hooks, it is necessary to assess the effect of a specific bait across all taxa directly or indirectly affected by a particular gear type before adopt- Globally, shark populations are widely et al. (2012) reported a higher catch of ing any bycatch reduction measure. reported to be in decline (for a recent blue sharks on mackerel (Scomber spp.) review, see Dulvy et al., 2014). Although bait than on squid bait. All 3 studies a number of factors contribute to this used a single bait type on each longline trend, bycatch of sharks is a primary set. However, none addressed potential factor (Oliver et al., 2015). A number biasing factors of this experimental of potential shark bycatch mitigation design in assessing bait preference of measures have been considered, includ- blue sharks, most notably the propen- ing time or area closures, gear modifi- sity for this shark species to form large cations, adaptive fishing practices, and aggregations in discrete locations (e.g., use of repellents (Molina and Cooke, Kenney et al., 1985; Litvinov, 2006) 2012). Several studies have investigated where only a single bait type would the efficacy of bait type in reducing have been used. shark bycatch rates (e.g., Broadhurst Driggers et al. (2017) examined catch and Hazin, 2001; Watson et al., 2005; rates and bait preferences of fish spe- Yokota et al., 2009); however, results cies captured on bottom longline gear of studies examining the relationship in the northern Gulf of Mexico using an between catch rates and bait type on experimental design alternating 2 bait longline gear are often conflicting. For types on individual bottom longline sets. Manuscript submitted 25 March 2019. example, in studies with comparable The results of this study indicate that Manuscript accepted 24 July 2019. gear comparing squid and mackerel several shark species, including the lit- Fish. Bull. 117:189–195 (2019). Online publication date: 1 August 2019. baits, Watson et al. (2005) and Foster tle gulper shark (Centrophorus uyato), doi: 10.7755/FB.117.3.6 et al. (2012) found that catch of blue smooth dogfish (Mustelus canis), blac- sharks (Prionace glauca) was highest knose shark (Carcharhinus acronotus), The views and opinions expressed or on pelagic longlines when squid (Illex sandbar shark (C. plumbeus), and Atlan- implied in this article are those of the spp.) was used compared with when tic sharpnose shark (Rhizoprionodon author (or authors) and do not necessarily reflect the position of the National bait of Atlantic mackerel (Scomber terraenovae), showed a preference for Marine Fisheries Service, NOAA. scombrus) was used, whereas Coelho Atlantic mackerel over northern shortfin 190 Fishery Bulletin 117(3) squid (Illex illecebrosus). Conversely, tele- osts commonly targeted with bottom long- line gear in the northern Gulf of Mexico, such as red snapper (Lutjanus campecha- nus), yellowedge grouper (Hyporthodus flavolimbatus), and tilefish (Lopholatilus chamaeleonticeps), showed no preference. The authors acknowledged that the use of alternating bait types could have biased results because of potential interaction effects of baits, a possibility suggested by Watson et al. (2005) and Foster et al. (2012). To address this issue and fur- ther investigate the efficacy of bait type in reducing shark bycatch, the objective of this study was to assess if the prefer- Figure 1 ence of sharks for Atlantic mackerel over Map of sampling locations where bottom longline gear was set in the northern northern shortfin squid baits reported by Gulf of Mexico during March–April 2017. Crosses indicate longline sets baited Driggers et al. (2017) would be evident in with northern shortfin squid (Ilex illecebrosus), and circles represent long- a single- bait experimental design. line sets baited with Atlantic mackerel (Scomber scombrus). Gear was set in waters off Louisiana (LA), Mississippi (MS), Alabama (AL), and Florida (FL). Gray lines indicate the 10-, 20-, 30-, 40-, 50-, and 100- m isobaths. Materials and methods Bottom longline gear was deployed from the NOAA Ship captured organisms and to maximize the number of long- Oregon II in the northern Gulf of Mexico from 26 March line sets conducted during the allotted sampling period. through 19 April 2017. All sets were made within an Further, the relatively short soak time reduced data loss area bounded by 87.50°W to 93.00°W longitude at depths associated with shark depredation and potential bias asso- between 15 and 85 m (Fig. 1). Stations were preselected ciated with capturing large sharks feeding upon smaller on the basis of a random starting point and subsequent captured fish, a behavior that would not reflect prefer- stations being spaced 18.5 km apart in a grid to maxi- ence for Atlantic mackerel or northern shortfin squid. mize sampling effort and minimize effects of sampling Upon capture, each fish was identified to species and its sites being in close proximity to one another. On several fork length was measured to the nearest millimeter on a occasions, preselected stations were moved to avoid haz- straight line along the axis of the body from the tip of the ards to navigation (e.g., vessels, petroleum platforms, rostrum to the caudal notch. and shipping lanes) or excessively fast currents (>1 m/s). Depth, temperature, salinity, soak-time , and fish- length Sampling gear consisted of 1.8 km of 4- mm diameter data associated with sampling locations where each bait monofilament mainline and 100 gangions. Each gangion was used were assessed for normality and homoscedas- was constructed of a size 148-8/0 snap, 3.2 m of 3- mm ticity by calculating kurtosis, skewness, and F- ratio. Data diameter monofilament, 0.5 m of 2.4-mm diameter fish- were considered normally distributed when kurtosis and ing wire, and a hook (#15/0 non- offset, circle, Mustad skewness were between −2 and 2. For data that adhered #39960D1, O. Mustad & Søn A.S., Gjøvik, Norway) and to the assumptions of parametric statistics, t- tests were was identical to that used in Driggers et al. (2017). Each used to compare mean values; for data that did not meet hook was baited with Atlantic mackerel or northern assumptions, median values were compared by using a shortfin squid cut to fit the circle hooks. Bait consisted Mann–Whitney W test. To determine if the number of indi- either entirely of Atlantic mackerel or northern shortfin viduals captured on each bait was significantly different squid for each longline set with bait randomly assigned from 1:1, a chi- square test with Yates correction for con- at the beginning of each set. tinuity was used on a species- specific basis. Sharks that Bottom temperature (in degrees Celsius) and salinity were captured on hooks occupied by a smaller captured were measured at each sampling location, while the long- fish were removed from all analyses (2 blacktip sharks, line gear was in the water, by using an SBE 911plus CTD Carcharhinus limbatus), and fish not measured because of water profiler (Sea-Bird Scientific, Bellevue, WA). Soak shark- inflicted damage were excluded from comparisons time, defined as the time elapsed from completing deploy- of body length. ment of the gear to beginning retrieval of the gear, was To account for the possibility that catch data were limited to 1 h in an effort to minimize mortality rates of biased because of schooling behavior or areas of high abundance (i.e., high catch in limited cases because of habitat specificity or aggregations), catch data for each 1 Mention of trade names or commercial companies is for identi- fication purposes only and does not imply endorsement by the species were examined to determine if an unusually high National Marine Fisheries Service, NOAA. number of individuals were caught on any specific set. Driggers and Hannan: Efficacy of 2 common bait types in reducing bycatch of coastal sharks 191 The mean number of a given species captured on all sets, All captured fish had corresponding body length mea- regardless of bait, was calculated. All sets where the surements with the exception of 23 specimens damaged total number of individuals captured was greater than by sharks (14 red snapper and 9 Atlantic sharpnose 3 standard deviations from the mean were considered sharks).