Demographic and Life History Characteristics of Black Bullheads Ameiurus melas in a North Temperate USA Lake

Black bullheads Ameiurus melas are an environmentally tolerant omnivorous fish species that are found throughout much of North America and parts of Europe. Despite their prevalence, black bullheads are an infrequently studied species making their biology, ecology, and life history poorly understood. Although limited information has been published on black bullheads, evidence suggests that bullheads can dominate the fish biomass and have profound influences on the fish community in some north temperate USA lakes. The goal of our study was to provide additional information on black bullhead population demographics, growth rates, life history characteristics, and seasonal diet preferences in a northern Wisconsin lake. Using common fish collection gears (fyke netting, electrofishing), fish aging protocols, fecundity assessments, and diet indices, our results suggested that black bullheads exhibited relatively fast growth rates, early ages at maturity, moderate fecundity, and a diverse omnivorous diet. Due to these demographic and life history characteristics, black bullheads have the potential to dominate fish community biomass in their native and introduced range. Results from our study may inform the management of black bullhead as native and invasive species.

Non-pharmaceutical interventions and the emergence of pathogen variants

Non-pharmaceutical interventions (NPIs), such as social distancing and contact tracing, have been widely implemented during the COVID-19 pandemic. In addition to playing an important role in suppressing transmission, NPIs influence pathogen evolution by mediating mutation supply and altering the strength of selection for novel variants. However, it is unclear how NPIs might affect the emergence of novel variants of concern that are able to escape pre-existing immunity (partially or fully), are more transmissible, or cause greater mortality. Here, we analyse a stochastic two-strain epidemiological model to determine how the strength of NPIs affects the emergence of variants with similar or contrasting life-history characteristics to the wildtype. We show that, while stronger and timelier NPIs generally reduce the likelihood of variant emergence, it is possible for more transmissible variants with high cross immunity to have a greater probability of emerging at intermediate levels of NPIs. However, since one cannot predict the characteristics of a variant, the best strategy to prevent emergence is likely to be implementation of strong, timely NPIs.

Validating morphological condition indices and their relationship with reproductive success in great-tailed grackles

Morphological variation among individuals has the potential to influence multiple life history characteristics such as dispersal, migration, reproductive success, and survival (Wilder et al. 2016). Individuals that are in better “condition” can disperse or migrate further or more successfully, have greater reproductive success, and survive longer (Heidinger et al. 2010; Liao et al. 2011; Wilder et al. 2016), particularly in years where environmental conditions are harsh (Milenkaya et al. 2015). Body condition is defined in various ways, but is most often measured using an individual’s energetic or immune state (Milenkaya et al. 2015). These traits are difficult to measure directly, therefore a variety of morphological proxies to quantify condition are used instead, including fat score (Kaiser 1993), weight, ratio of weight to tarsus length (Labocha et al. 2014), a scaled mass index (Peig and Green 2009), as well as hematological indices for immune system function (Fleskes et al. 2017; Kraft et al. 2019). However, there is mixed support regarding whether these condition indices relate to life history characteristics (Labocha et al. 2014; Wilder et al. 2016), and whether the relationship is linear (McNamara et al. 2005; Milenkaya et al. 2015). Additionally, although some investigations use multiple morphological proxies for condition (e.g. Warnock and Bishop 1998), rarely have there been direct comparisons among proxies to validate that they measure the same trait. In this investigation, we define condition as an energetic state and we attempt to measure it by comparing two indices (fat score and the scaled mass index) to validate whether they measure the same trait and whether they correlate with measures of reproductive success in our study system, the great-tailed grackle (Quiscalus mexicanus). We found that the morphological proxies did not correlate with each other, indicating that they do not measure the same trait. Further, neither proxy correlated with reproductive success in males, measured as whether a male held a territory containing nests or not. We found that females with a high scaled mass index had a significantly lower probability that their nest would survive on any given day. However, there was no relationship between female fat score and nest survival. These results indicate that measures of condition should be validated before relying on their use as a condition proxy in grackles and birds in general. Future research should further investigate our unexpected result that higher scaled mass index correlated with lower nest survival to better understand the importance of energetic condition for reproductive success - a necessary component for selection to act.