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Midwater Assessment and Conservation Engineering

Research To Reduce Crab Mortality from Trawl Footropes and Improve Tools for Near-bottom Walleye Pollock Capture

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July-Sept 2012
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Conservation engineering scientists with the Center’s conducted research aboard the chartered fishing vessel Great Pacific 1 – 21 September continuing studies to reduce effects of fishing gear on Bering Sea crab resources. During the cruise, scientists measured and assessed thousands of Tanner and snow crabs that had contacted bottom trawl footropes to estimate their mortality rates and to see whether that rate would be reduced by changes to footrope design. Crabs were captured after footrope encounters with nets suspended under the trawl (Fig. 1).

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Figure 1.  Crabs being sorted from catches of special nets fished behind trawl footropes.

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Figure 2.  Crab being measured and assessed for reflex impairments.


Assessments for reflex impairments were used to evaluate the probability of delayed mortality (Fig. 2). This work was funded by the North Pacific Research Board and continued cooperative research efforts with the Bering Sea bottom trawl fleet to reduce effects on Bering Sea crabs. An earlier result of these efforts was changes to trawl sweeps that are now used during all flatfish trawling in the Bering Sea. Those sweep changes eliminate almost all direct seafloor contact, reducing crab mortality and effects on seafloor habitat. Footrope changes tested on the Great Pacific also emphasized reducing direct seafloor contact to the extent possible while maintaining their function in capturing fish.

The second objective of this research cruise was an initial study of alternative trawl groundgear for capturing pollock concentrated near the seafloor.   Pollock trawlers are required to use pelagic trawls to capture pollock. However, when pollock concentrate near the seafloor, fishing nets designed to capture fish in midwater and towed along the seafloor can cause both inefficient capture and the potential for bycatch and damage to benthic organisms.

Scientists made initial observations of alternative sweep and footrope configurations designed to achieve effective pollock herding and capture, while minimizing seafloor contact and effects on benthos. Observations included the proportion passing under different footrope designs and herding behavior ahead of raised sweeps.

Fish catches during this cruise were passed through a video assessment system. This device is being tested as an alternative method for rapidly identifying and measuring fish catches. This simple system consists of an enclosure with lighting and a high-resolution camera/recorder, mounted over an inclined chute. Automated tools are being developed to analyze the resulting video, providing species and size for each recorded fish.

By Craig S. Rose

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