Gill nets are commonly used to sample Alligator Gar Atractosteus spatula; however, gill nets are size selective. We used five common multifilament gill‐net meshes (88.9‐, 101.6‐, 114.3‐, 127.0‐, and 139.7‐mm bar measure) to collect 477 gar between 1,000 and 2,300mm TL. We fit size‐selectivity functions for these meshes for a net in which each mesh was fished equally, i.e., equal‐effort net design (EEND), and found selectivity was best described by a binormal distribution in which the assumption of geometric similarity had been relaxed (i.e., binormal with deviations). Overall relative retention was about 30% for fish at 1,200 mm, increased to 100% for fish at 1,670 mm, then declined to 50% when fish were 2,100 mm. To reduce size selectivity, we used an optimization to create a net that minimized bias over the greatest TL range of fish sampled, i.e., weighted‐effort net design (WEND). We compared the WEND to the EEND via simulation to estimate the effects of selectivity bias on population metrics. The WEND had 70% or greater relative retention for all fish between 1,200 and 2,100 mm TL. While the WEND performed slightly better with respect to the length frequency, simulations indicated that sampling Alligator Gar populations with either net design led to estimates of vital statistics for fish above 1,200 mm that were only slightly biased (e.g., survival estimates were biased by <2%). Both designs underestimated the proportion of smaller fish (i.e., <1,200 mm), which affected estimates of most vital statistics for these fish. When the goal of sampling is to estimate vital parameters for Alligator Gar < 1,200 mm TL, biologists should consider using different mesh sizes or methods. Bias adjustments for larger and older fish are probably unnecessary. Our results suggest the gill‐net meshes we tested can be used to collect a representative sample of adult Alligator Gar. Received October 29, 2015; accepted February 2, 2016 Published online May 26, 2016
Management of the Alligator Gar Atractosteus spatula has been hampered by the lack of sampling methods that can efficiently reveal population characteristics or minimize incidental sampling mortality. In a 3-year evaluation of the Alligator Gar population in Choke Canyon Reservoir, Texas, we documented changes in sampling methodology that produced an eightfold increase in multifilament gill-net catch rates (from 0.66 to 5.10 fish/h). The increased sampling efficiency was attributed to the development of an aggressive-predator sampling strategy over the study period; our highest catch rates were achieved by (1) observing surfacing fish before net deployment and (2) relocating to a new sampling site when catch rates were less than 1 fish/h. This aggressive strategy resulted in shorter net soak times, which reduced incidental net mortalities; nets soaking less than 30 min produced zero mortalities. Our results suggest that the aggressive-predator strategy is suitable for the majority of Alligator Gar sampling objectives (e.g., collection for age and growth and mark-recapture), which will improve the ability to manage Alligator Gar populations.
The Texas Parks and Wildlife Department’s strategic management plan for catfishes (Ictaluridae) called for the development of alternative harvest regulations for Blue Catfish Ictalurus furcatus and Channel Catfish I. punctatus. Angler harvest of these species in most Texas reservoirs has been regulated using a 305‐mm minimum length limit (MLL) and a 25‐fish aggregate daily bag limit (DBL) since 1995. Using existing creel data collected in 2003–2018 at 86 Texas reservoirs managed under this statewide harvest regulation, we examined Blue and Channel Catfish harvest size structure and composition, percent release of legally harvestable‐size fish, and harvest density (number and biomass). Additionally, for reservoirs having long‐term creel data, we evaluated temporal change in percent release of legally harvestable‐size fish and harvest size structure, and we estimated the impact of alternative harvest regulations by using a theoretical harvest reduction (THR) model. Channel Catfish were the predominant species harvested (>75% of harvest) in 63% of the reservoirs. Harvest size structures of Blue and Channel catfish differed, but the difference in percent harvest of fish smaller than 406 mm was negligible (52% and 48%, respectively). Percent release of legally harvestable‐size fish (≥305 mm) was low at most reservoirs (median = 13%). Total number of catfish harvested per hectare per quarter (HHQ) ranged widely (0.0–11.3) but was less than 1.0 at 58% of reservoirs. Harvest size structure was truncated over time coincident with a high harvest level (HHQ > 3.0) at two of four reservoirs with long‐term data, suggesting growth overfishing. The THR model revealed that increases in MLL resulted in variable harvest reductions among reservoirs and that removal of the 305‐mm MLL could increase harvest by 6–39%. Additionally, we found that length‐graduated DBLs would have a negligible impact on reducing the harvest of larger fish. We suggest that MLLs are not necessary for most Texas catfish fisheries and that a 356‐mm MLL may be appropriate for high‐harvest fisheries.
We evaluated the efficacy of trap nets fitted with a batteryoperated, submersible light in the net mouth in increasing the catch rates of white crappies Pomoxis annularis by comparing the catch rate and size structure of white crappies collected by these nets with those of fish collected by standard (i.e., unlighted) trap nets in six Texas reservoirs. The catch rates of standard (mean = 16.3/netnight) and lighted trap nets (12.1/net-night) did not differ significantly. We found no significant difference in the catch of stock-size white crappies (≥130 mm total length [TL]) by standard (12.7 fish/net-night) versus lighted trap nets (9.9 fish/net-night); however, significantly fewer preferred sizes (≥250 mm TL) were taken by lighted (1.9 fish/net-night) than by standard sets (2.9 fish/netnight). The length-frequency distributions of the white crappies collected by each trap net treatment were similar and did not significantly differ in four of the five lakes tested. Our results suggest that trap nets fitted with artificial lights in the net mouth do not effectively increase catch rates for white crappies and may favor the collection of smaller size-classes than standard trap nets.
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