Deep-Water Electroshocker for Sampling Small Fishes and Invertebrates from Interstitial Spaces

Sampling in deep-water habitats poses challenges for researchers studying small-bodied, cryptic species.  For example, assessment of lake trout Salvelinus namaycush reproductive success has recently focused on deep, offshore reefs where impediments to survival of eggs and fry, such as habitat degradation and predation by round gobies Neogobius melanostomus, are less severe than in nearshore habitats.  Lake trout fry remain in interstitial spaces on spawning reefs for 3-4 weeks after hatching until they emerge and disperse into deeper water; thus, they are challenging to sample with conventional methods.  We developed an electroshocker, mounted on a remotely-operated vehicle (ROV), to attract fry out of the substrate and make them accessible to a camera for visual assessment and to a suction sampler attached to the ROV.  A simpler, drop-electroshocker, mounted on a PVC frame with a video camera, was tested in the laboratory and on shallow reefs in Lake Champlain to quantify size and strength of the electric field and susceptibility of the fry to the electroshock.  In laboratory tests, a field strength of 0.2 V/cm stimulated fry movement, making them highly visible; at 0.5 V/cm fry became immobilized. The electroshocker detected fry in 90% of laboratory trials and 34-56% of field trials (using on-site emergence traps as a standard for presence). Fry CPUE using the electroshocker was proportional to fry density in the laboratory and fry CPUE from emergent fry traps.  We then used the ROV-mounted shocker to sample fry at two sites > 35 m depth within the Mid-Lake Reef Complex (MLRC) in Lake Michigan. During this study we discovered that the electroshocker and suction sampler also sampled Mysis diluviana effectively.  Mysis densities on the lake floor were measured by triggering the shocker while the ROV was sitting at rest on the bottom; Mysis in motion stimulated by the electroshocker were then counted within a known area in the ROV’s field of view while reviewing the video in the lab.  Using the ROV’s tracking system, distribution and densities of Mysis at the MLRC could then be assessed.  Both the ROV-mounted and drop-shocker extend fish and invertebrate sampling capabilities to water depths that were relatively inaccessible to researchers except by scuba, and have therefore been historically under-sampled.