Time: 12:18 AM. Date: 13 Aug, 2008. Location: River Mile 177, Missouri River, USA.  Significance: HAMP collects nocturnal pallid sturgeon with push trawl.

Near Columbia, Mo., at a large channel sandbar locally known as California Island, HAMP went nocturnal to claim the first pallid sturgeon collected with a push trawl on lower Missouri River.  HAMP’s purpose was to determine whether sturgeon were moving into shallow water areas after-hours. This catch is notable because HAMP has push trawled for the last three years with a total of over 2,500 gear deployments during the day with out a single pallid sturgeon captured.  This fish was captured with fewer than 35 push trawl deployments over just three nights. The fish was stocked as a fingerling during the winter of 2003 in the channelized portion of the lower Missouri River and was marked with only a right-side pink elastomer tag on the ventral side of its snout.  It was 490 mm measured from the tip of its snout to the fork in its tail at the time of capture.

Young sturgeon collected in SWH at night with push trawl.

Using the push trawl at night, we have been able to capture multiple juvenile and young of year shovelnose sturgeon.  Few juvenile sturgeon - pallid or shovelnose - have been captured in areas meeting the criteria for shallow water habitat (SWH) as defined by the Biological Opinion.  However, a vast majority of the effort to sample these areas occurs during daylight hours.  Other large river fish species are known to seek deep water during the day then move into shallow areas during the nocturnal period.  For example, blue suckers are seldom captured in SWH with the push trawl during the day, but several were captured during the night effort.  Several hypotheses exist among scientists to explain why some fish migrate towards shallow areas at night.  One that we are interested in testing is that large predatory fishes move shallow at night to hunt prey.  This seems especially relevant to pallid sturgeon because they had adapted to life in the very turbid Missouri before upstream dams were closed causing the river to become much clearer.  If pallid sturgeon are photophobic (i.e., avoid clear, shallow areas during the day), their only option is to access this feeding area at night.  However, several pertinent factors affecting this hypothesis are merely speculation at this time; pallid sturgeon phototaxis and diet have yet to be studied on lower Missouri River.  The extra sampling effort put forth by HAMP crews at night is the preliminary step to scientifically answering important questions to aid sturgeon recovery.

Fish Biologist Clayton Ridenour with adult blue sucker collected in SWH with push trawl at night.

Currently, night push trawling effort is limited in scope and resources, and no formal analyses have been conducted on the data collected.  However, the concept that fish shift habitats between day and night is well supported in the scientific literature.  Since the nocturnal period represents approximately half of a sturgeon’s life span, it is worth while to address diel habitat use in light of SWH restoration.  In short, most of the data on sturgeon habitat use in lower Missouri River accounts for only half of the story.

Our efforts to further understand the complex interactions of fish and their association with shallow water habitat supports both the “Leadership in Science and Technology” and “Aquatic Habitat Conservation and Management” goals of the Fisheries Program’s Vision for the Future.

Clayton Ridenour and Andy Starostka