Densities, Diets, and Growth Rates of Larval Alewife and Bloater in a Changing Lake Michigan Ecosystem

Abstract

Variability in abiotic and biotic factors during larval stages has profound impacts on fish recruitment. In Lake Michigan, where the composition of lower trophic levels has undergone considerable changes in the past decade, managers are concerned that fish recruitment could be negatively affected. We hypothesized that spatial variation in Lake Michigan larval fish density and growth can be explained by various environmental predictor variables. In July 2015, we sampled larval fish and zooplankton at 24 sites (distributed among eight transects) around Lake Michigan. We measured larval fish densities and estimated growth rates and diets of the two most abundant species: the Alewife Alosa pseudoharengus and Bloater Coregonus hoyi (prey fish that represented 89% and 4% of the total catch, respectively). Larval Alewife densities at a given site ranged from 0 to 42.57 larvae/100 m3, but no explanatory variables explained the variation. Alewife mean growth rate equaled 0.50 mm/d, and fish age and zooplankton density best explained growth variation across sites. Larval Bloater densities ranged from 0 to 1.16 larvae/100 m3, and mean growth rate was 0.21 mm/d. Across all sites, 67% of larval Alewife stomachs were empty, whereas only 16% of Bloater stomachs were empty. Larval fish growth rates observed in our study were at least 40% slower than those reported in previous decades for both Alewife and Bloater. Worsening prey environment for pelagic larvae, such as Alewife and Bloater, during the era of abundant dreissenid mussels could reduce the probability of strong year‐classes, which in turn may affect growth and survival of recreationally important salmonine predators.

Predicting larval alewife transport in Lake Michigan using hydrodynamic and Lagrangian particle dispersion models

“…An average‐sized alewife in our 2015 collections (i.e., 12 mm) likely only swims at a speed of 0.02–0.04 m s −1 (Klumb et al 2003). These results suggest that larval alewife are highly susceptible to transport by currents in Lake Michigan, and that the collection of alewife larvae in offshore waters (Weber et al 2015; Eppehimer et al 2019) was most likely the result of advection from nearshore or riverine spawning habitats.…”

“…Given our predictions of alewife transport, future research should duplicate our methods in other years to determine whether 2015 was an unusual year with respect to its prevailing winds and resultant currents (alongshore, upwelling, downwelling) or whether the proximity to the coastline of hatching is a more generalizable result. Furthermore, this approach could be coupled with a biological model that uses backward trajectories to predict zooplankton prey densities and temperatures experienced each day of “drift” to try and explain observed differences in growth rates and densities of larval alewife (see Eppehimer et al 2019). Understanding the drivers of variation in recruitment for alewife remains a priority for fishery managers that adjust stocking rates of recreationally and commercially important salmonine species based on alewife recruitment and resultant population biomass.…”

“…We used sagittal otoliths to estimate the age of larval alewife as these are considered to be the most precise structure for this type of analysis (LaBay and Lauer 2006). Daily otolith growth increments have been previously used as a proxy for alewife age (Höök et al 2007; Eppehimer et al 2019). The first ring is likely laid on the second day of life (Essig and Cole 1986); thus, we added two to the number of increments counted to determine the age of the larval fish.…”

“…We conducted blind age estimates and compared them to those reported by Eppehimer et al (2019). If our age estimate differed by more than 1 d from Eppehimer et al (2019), a second analyst counted the rings, and if the estimate from the second analyst differed by more than a day from the reading by Eppehimer et al (2019), the sample was omitted from the study.…”

“…Shortly thereafter, Chinook salmon populations collapsed (Johnson et al 2010), with very limited evidence of a recovery (Clark et al 2017). To protect Lake Michigan against a similar outcome, managers seek to match the predator demand of salmonids with variable alewife population that may also be influenced by changes in lower trophic levels (Bunnell et al 2018; Eggold et al 2018; Eppehimer et al 2019). Understanding the mechanisms underlying variable alewife recruitment would help both inform agency stocking decisions and improve stakeholder understanding of why stocking may need to be modified.…”