Upper Mississippi River fishes grow in varied proportions

Fish in the Upper Mississippi River Basin generally become plumper or more elongated as they grow, according to a study that quantified length–weight relationships for 14 native minnow and sucker species using three decades of federal monitoring data. Researchers fitted separate regressions to 7,731 fish and found that nine species departed from the isometric expectation that weight increases in direct proportion to the cube of body length. The findings provide a species-specific foundation for estimating biomass, filling missing measurements and comparing the condition of fish from different parts of this ecologically and economically important river system.

The study analyzed daytime boat-electrofishing records collected by the Upper Mississippi River Restoration–Environmental Management Program’s Long Term Resource Monitoring Program. The archive covered 1989 through 2022 and included samples from six navigation reaches: five on the Mississippi River and one on the Illinois River. After records were parsed and edited, unidentified specimens and young-of-the-year fish were removed, along with 935 potential outliers representing about 12% of the original records. Individual total length was recorded to the nearest millimeter and converted to centimeters, while wet body mass was measured from 0.01 to 1 gram.

The dataset initially contained 682,982 collection records and roughly 1.9 million individual fish or groups representing 159 species. The researchers selected 14 members of the order Cypriniformes: four North American minnows in the family Leuciscidae and 10 suckers in Catostomidae. Species were chosen where the monitoring archive contained more than 30 usable observations, the animals were native to the basin and the available length–weight information was limited or based on studies from other populations. Final sample sizes ranged from 21 Mimic Shiners to 2,813 Smallmouth Buffalo.

To estimate the relationship between length and weight, the investigators transformed both variables to base-10 logarithms and used ordinary least-squares regression, the standard approach for fish length–weight models. The fitted relationship was expressed as log weight equal to log a plus b multiplied by log length, with length measured in centimeters and weight in grams. Here, the intercept parameter a and slope parameter b describe the fitted curve, while 95% confidence intervals were calculated from 1,999 permutations. A two-tailed t-test evaluated whether each slope differed significantly from 3, the value expected under isometric growth, at an alpha level of 0.05.

All 14 regressions were statistically significant at P less than 0.001, and length explained more than 95% of the variation in weight for 13 species. The exception was the Red Shiner, whose adjusted R-squared value was 0.854, possibly reflecting its relatively small sample of 54 fish or unmeasured recording errors. Across the species, intercept estimates ranged from 0.00220 for the Blue Sucker to 0.014 for the Highfin Carpsucker, while slopes ranged from 2.834 for the Mimic Shiner to 3.351 for the Blue Sucker. The largest fish was a Black Buffalo measuring 89.0 cm and weighing 10,566 g; the smallest maximum was a 6.5-cm Mimic Shiner weighing 2.1 g.

Slopes were greater than 3 in 10 of the 14 species, or about 71%, and t-tests rejected isometric growth in nine. Those nine were the Emerald Shiner, Mimic Shiner, River Carpsucker, Blue Sucker, Smallmouth Buffalo, Black Buffalo, Silver Redhorse, Golden Redhorse and Shorthead Redhorse. Their estimated growth patterns were weak rather than extreme, with every slope remaining between 2.5 and 3.5, a range generally considered typical across diverse fish assemblages. Slopes above 3 indicate positive allometry, in which larger fish gain weight faster than expected from length alone; slopes below 3 indicate negative allometry, in which length increases faster relative to mass.

The Emerald Shiner was the clearest negative-allometry example, with a slope of 2.894 and a 95% confidence interval of 2.851 to 2.938. Its slender, small-headed body form is consistent with larger individuals adding relatively more length than girth. Six of the species with significant departures from isometry instead had slopes above 3, indicating positive allometry. These included the River Carpsucker, Smallmouth Buffalo and Black Buffalo, which have deeper bodies than the redhorse suckers, as well as the Silver Redhorse, Golden Redhorse and Shorthead Redhorse. The researchers suggested that the deeper-bodied fishes add girth and body depth as they develop, although the statistical results establish the growth pattern rather than directly measuring the anatomical processes behind it.

Five species did not show statistically significant departures from isometry: Red Shiner, Spottail Shiner, Quillback, Highfin Carpsucker and River Redhorse. The confidence intervals for four of those slopes included 3.0, but the interpretation remained cautious because each had fewer than 100 specimens. Among them, the Highfin Carpsucker alone met the study’s full criteria for confidently interpreting growth as isometric. Its 2018 capture established a new maximum reported total length of 51.4 cm and weight of 2.1 kg. The other four relationships were described as provisional, and the authors advised that missing weights, if estimated, should be imputed only within the length classes represented by the samples.

Several limitations constrain how widely the models can be applied. Boat electrofishing generally favors smaller fish and species associated with shallower habitats and riverbanks, so rare taxa and large individuals may be underrepresented. The most limited relationship, for Mimic Shiner, was based on only 21 fish. Lower-order tributaries, lakes and additional sampling gears could provide data missing from the main-channel records and allow more robust comparisons. The monitoring reaches also vary in floodplain characteristics, geomorphology, navigation infrastructure and surrounding land use, and the study pooled these environments rather than estimating a separate relationship for every reach, habitat or season.

The new estimates generally agreed within roughly 2% to 5% with previously available FishBase relationships, but those earlier models often came from different populations or geographic regions. The present study substantially expands the evidence for six species that had only one prior length–weight record: all four minnows, the Blue Sucker and the Black Buffalo. The former Blue Sucker estimate, for example, was based on six specimens, while the Black Buffalo relationship had been calculated from a single world-record fish. Models based on 54 Blue Suckers and 522 Black Buffalo spanning multiple size classes provide more defensible regional parameters. The results therefore turn an unevenly documented collection of relationships into a broader, locally grounded set of tools for estimating fish weight and biomass in the Upper Mississippi River Basin.

Subject of Research: Length–weight relationships and allometric growth in 14 Upper Mississippi River cypriniform fish species

Article Title: Length–weight relationships for 14 cypriniform freshwater fish species ( Actinopterygii ) from the Upper Mississippi River Basin

Article References: Bagley, J. C., & Breitman, M. F. (2024). Length–weight relationships for 14 cypriniform freshwater fish species (Actinopterygii) from the Upper Mississippi River Basin. Acta Ichthyologica et Piscatoria, 54, 213-220. https://doi.org/10.3897/aiep.54.126623

Image Credits: AI Generated

DOI: 10.3897/aiep.54.126623

Keywords: length–weight relationships, allometric growth, Upper Mississippi River, cypriniform fishes, minnows, suckers, fisheries science, fish biomass, electrofishing, environmental monitoring, Length, weight