High water, spillway openings take toll on Louisiana harvests

High-water events and the unprecedented two spring 2019 openings of the Bonnet Carré Spillway have sharply depressed Louisiana’s landings of oysters, shrimp, crab and finfish, the state Department of Wildlife and Fisheries reported this week.

Flood levels across the Mississippi River basin and associated rivers have been playing havoc with communities, farmers and the inland barge industry. High water has been so persistent that the Army Corps of Engineers has opened the Bonnet Carré Spillway – a control structure that diverts Mississippi River waters to protect New Orleans from flooding – twice this year.

The structure was opened from 27 February to 11 April, sending water down the dedicated spillway to Lake Pontchartrain, and then again in mid-May as a second high water crest came down the river.

It was the first time the structure was used twice in one year, and the first time it has been opened two years in a row since it was completed in 1931 in response to the devastating 1927 high water.

All that fresh water, drained from 41 percent of the continental United States, has tremendous effect on fisheries when diverted out of the river.

Hardest hit were oysters harvested from public bottoms, with landings down by 80 percent from the year-to-day average, according to commercial harvest data as of 6 June, said DWF officials. Those landings were 89 percent below average during the months of March and April, while landings from private reefs dropped 40 percent.

Statewide landings of brown and white shrimp declined 36 percent in March and 45 percent in April, compared to the five-year average. Biological sampling in the Vermilion/Atchafalaya basin, an area known for white shrimp, showed an 85 percent decrease in shrimp numbers, and brown shrimp catches showed a 66 percent decline from long-term averages.

That sampling result “strongly suggests that commercial landings will be far below long-term averages,” the agency warned. Biological sampling in bays and marshes of St. Bernard Parish during May showed brown shrimp numbers far down, almost four times lower than the five-year average.

Blue crab landings across Louisiana were down by 33 percent during March and 45 percent in April compared to the five-year average. State biologists found April blue crab sampling results in the Pontchartrain basin – where the Bonnet Carré diverts the Mississippi high water releases – were 60 percent below long-term averages. In the Vermilion/Atchafalaya basin, sampling results were 78 percent below the long-term average.

Biologists are collecting fewer spotted seatrout in the Vermilion/Atchafalaya, as they expected with the timing of high water and the low salinities it brings – at a time when females trout are seeking high-salinity waters to spawn.

Other finfish impacts include a sharp drop in black drum landings, down 53 percent in the Vermilion-Tech basin and 43 percent in the Calcasieu basin; overall commercial black drum landings are down 31 percent year to date.

It’s not an official disaster yet. Statewide species declines of 35 percent, the threshold for federal fisheries disaster declarations, have yet to be see.

But “the true impact of the spillway opening and other high water events on fish and animals won’t be known for months as data continues to be collected and analyzed by both state and federal agencies,” according to the department.