What’s the Hatch this Year

“What’s the hatch this year?”  It is one of the most common questions Division of Wildlife fish biologists hear from the public besides requests for fishing information. 

Survey crews from the Division of Wildlife’s two Lake Erie fish research stations annually evaluate hatches of walleye and yellow perch. Assessment surveys using bottom trawls on Division Research Vessels Explorer and Grandon are completed at up to 32 stations in the Ohio waters of the western basin (Toledo to Huron) and 64 stations in the central basin (Vermilion to Conneaut). The bottom trawl surveys are performed at randomly-chosen stations, but at specific water depths, to accurately portray the conditions across the basins. Surveys are performed in August when the lake is thermally stratified with a well-defined thermocline, and again in mid-September to early October, after the lake turns over in the fall and the thermocline breaks down.

Research vessels Grandon (left) and Explorer (right) are docked and ready.	Fisheries biologists perform a bottom trawl from a research vessel.

Knowing how these catches of young fish relate to the numbers of fish that show up in the fishery when they are older is vitally important. Predicting the success and contribution of each year’s hatch to the total number of fish in the lake allows managers to set the harvest quota and regulations with a great degree of certainty. 

Biologists use the term recruitment to describe the number of fish of a certain age, year class, or size entering a fishery. Juvenile fish catches are used from the trawls to project populations at older ages because recruitment of yellow perch and walleye to older ages is set relatively early in life. In other words, big catches of juveniles generally equates to a large number of adult fish from that year class and conversely, low catches of juveniles correspond to a poor number of adults from that year class. Generally, we use the catch rates of juveniles to predict the number of two-year old fish in Lake Erie. Two-year old fish estimates are used because that is when the fish approach harvestable size.

Adult fish caught by anglers, commercial fishing nets, and interagency survey assessments are used to estimate the populations of adult walleye and yellow perch in Lake Erie.  We then compare the estimates of the number of two-year old walleye and yellow perch in the population collected from the adult fish surveys to the number of juvenile fish captured in bottom trawls two years previous. The two different methods of evaluating the two-year old fish serve as validation to ensure we can accurately predict the total number of harvestable fish in the lake.

So now that you know the types of surveys and the procedures for verifying the data, I’m sure you still want to know “What’s the Hatch?” for 2011.  For walleye, the August trawl survey resulted in a catch of 4.9 juvenile walleye per hectare; a rather below-average catch that will produce an estimated 7 million age-2 walleye which will begin to show up in the fishery in 2013.  For comparison, the average juvenile walleye catch since 1988 has been 29.9 fish per hectare, which results in just over 11 million walleyes recruited annually. The large 2003 walleye cohort produced a catch rate of 155.6 fish per hectare and recruited an estimated 68.8 million fish. 

For yellow perch, the average August trawl survey catch in the western basin was 29.9 fish per hectare, which is the lowest index value recorded since 1987. For the west central sub-basin and east-central sub-basin, average August catches were also well below average at 8.6 and 12.6 fish per hectare, respectively. These below average central basin yellow perch catch rates will result in few fish recruiting in to the age-2 population and the fisheries in 2013. Catches of age-0 yellow perch in good year classes can reach over 1000 fish per hectare!  

Biologists use juvenile fish catches from the trawls to project populations at older ages.	For yellow perch, the average August trawl survey catch in the western basin was 29.9 fish per hectare.

The estimation techniques are not perfect, and there are places for uncertainty to enter into the equation, but they have been good indicators of future fishing success. The Division of Wildlife’s Lake Erie fisheries biologists continue to address techniques that will improve our ability to estimate juvenile and older fish, and increase precision and accuracy of our assessment models, thereby enhancing our efforts to manage the walleye and yellow perch populations and fisheries.