
As an angler, commercial netter, or observer of nature, have you ever wondered why some fish born in a given year, called a year-class, survive better than others?
Fisheries biologists and managers do, and they spend a lot of effort trying to determine year-class strength so they can predict survival of fish as they transition to larger, older life stages — a process called recruitment.
Why is recruitment important?
A better understanding of recruitment can help fisheries staff set more effective fish harvest limits.
Fisheries professionals test-drive a new model
A November 2024 workshop gave Minnesota, Wisconsin, and tribal fisheries managers and biologists an opportunity to test-drive a new “poor” recruitment model.

The "poor recruitment model" hypothesizes that it is easier to predict poor recruitment, which is when very few individuals survive to a certain age or size, rather than “good” recruitment, which is when many individuals survive to a certain age or size.
This is because if only one of many environmental factors prevents fish survival, then that one factor can cause recruitment to be “poor” no matter the value of the other environmental variables. By contrast, “good” recruitment requires that none of the environmental factors are harmful, and that some need to be favorable for a strong year-class to be produced.
The poor-recruitment model was developed by Julie M. Gross, Philip Sadler, and John Hoenig at the Virginia Institute of Marine Sciences and appears in the August 2022 issue of the journal Fisheries Research.
“Understanding what causes large year classes and predicting them has been called the holy grail of fisheries science, one of the last great unanswered questions,” wrote Hoenig and colleagues in their paper “Evaluating a possible new paradigm for recruitment dynamics: predicting poor recruitment for striped bass (Morone saxatilis) from an environmental variable.” The paper was published in the journal Fisheries Research in 2022.
Traditionally, fish biologists focused on “good” or optimal environmental factors that influence fish survival such as temperature, windspeed, and precipitation to determine fish recruitment. It normally takes several of these “good” factors in combination to produce a strong year-class of fish.
By contrast, in most cases it takes only one factor to negatively influence recruitment.
While both approaches can provide important information on recruitment, the benefit of using the poor-recruitment model is that it is less complex to use because there is only one optimal environmental factor to be monitored. This model can save fish managers both time and money.
The workshop
On November 18-19, 2024, about 25 professional fisheries biologists and students from 10 different organizations gathered at the University of Minnesota St. Paul campus or joined online to explore and discuss how to use the poor-recruitment model in Minnesota and Wisconsin.

Oliva Nyffeler, a graduate student of Lynn Waterhouse and Gretchen Hansen at the University of Minnesota, adapted the poor-recruitment model to her Minnesota Sea Grant-funded project that was designed to determine recruitment of Ciscoe in Lake Superior. Nyffeler identified environmental variables that appear to negatively affect Cisco recruitment. The project and her master’s degree were largely supported by Minnesota Sea Grant.
With funding from both Minnesota Sea Grant and a UMN Moos Fellowship, Nyffeler, Waterhouse, and Hansen developed the Poor-Recruitment Paradigm Workshop. One of the goals of the workshop was to determine if the poor-recruitment model could be applied to a variety of fish species from lakes of different sizes.
Workshop participants came with data in hand to learn about the model requirements, manipulate their own data in the model, and test the model on various species and in various lakes. Participants tested Lake Trout in Lake Superior, Walleye in Lake Mille Lacs and several inland lakes, Yellow Perch in Leech Lake, and a variety of other species (e.g., Musky, Crappie, Bluegill, and others) from various inland lakes in both Minnesota and Wisconsin.
Participants found that ice-out dates and water temperature appeared to negatively influence fish survival (i.e., recruitment) for several species including Bluegill, Largemouth Bass and other panfish species. Panfish were likely negatively affected by late ice-out and cold spring water temperatures that delayed spawning and limited first-year growth which can affect overwinter survival.
Workshop participant and Wisconsin Department of Natural Resources, Office of Applied Science, Fisheries Research Team Leader Greg Sass said, “We’ve explored several ways to predict recruitment, mainly we have focused on what factors influence positive recruitment. This approach is interesting in that we are looking at our data in the opposite way, attempting to determine what influences negative recruitment.”
The Poor-Recruitment Workshop is an example of how Minnesota Sea Grant-funded research can deliver information and resources to stakeholders in a way that can help advance new applied science, train both professional biologists and students in using new techniques, and potentially assist fish managers in incorporating the results of a new model into their monitoring programs, setting harvest limits, and adjusting management plans.
“Applying a model from my thesis to predict fish recruitment for a species that is still yet to be understood, was incredibly exciting. Taking it a step further by sharing this method with professional biologists and collaborating to implement it felt truly rewarding,” said Nyffeler. “It was special to see how this outreach and work could contribute to real-world management and have a tangible impact.”
Workshop participants were surveyed in early December 2024 about whether they wanted to continue working on their species data sets with the goal of publishing a peer-reviewed paper that would address the applicability of the model to predict negative recruitment among a variety of species and locations in Minnesota and Wisconsin.
MORE INFORMATION:
Looking for information about Minnesota's fisheries? Check out Minnesota Sea Grant's Fisheries and Aquaculture Program project webpages and subscribe to Minnesota Sea Grant Monthly Highlights newsletter.
CONTACT:
Don Schreiner, Fisheries Specialist, Minnesota Sea Grant, University of Minnesota and University of Minnesota Duluth, [email protected].
Marie Thoms, Communications Manager, Minnesota Sea Grant, University of Minnesota and University of Minnesota Duluth, [email protected].