Drift versus retention: an alternative perspective to Wilde and Urbanczyk's ‘relationship between river fragment length and persistence of two imperiled great plains cyprinids’

Wilde and Urbanczyk (2013) supplemented a growing body of evidence that pelagic-broadcast spawning minnows (i.e., small-bodied cyprinids that produce non-adhesive, semi-buoyant eggs) require river fragments that are hundreds of kilometers long (Perkin & Gido 2011; Worthington et al. 2014). Their primary conclusion was that these fishes ‘depend on the presence of river fragments long enough to allow spawned ova and swim-up fry time to develop and move out of the current’. However, there is no empirical basis for the belief that pelagic propagules must drift as they develop. Wilde and Urbanczyk (2013) cite speculative conclusions of Moore (1944) and Platania and Altenbach (1998) for support, but Moore (1944) reared pelagic propagules in finger bowls, proving they do not need to drift to survive. Later, Bottrell et al. (1964) reared them in watch glasses and Hoagstrom (unpublished data) reared them in still-water aquaria (see Hoagstrom et al. 2006b).

Moore (1944) speculated that eggs must drift to develop because sand is unstable and too fine to provide interstices for incubation. He suggested pelagic eggs were vulnerable to burial or abrasion, but subsequent studies have shown that they are not. Near-neutral buoyancy makes eggs susceptible to very slight currents (i.e., ∼1 cm/s; Dudley & Platania 1999, 2007). Thus, they must be transported as part of the suspended load (sensu Einstein 1950) which precludes the risk of burial or abrasion by bedload sands. The settling velocity of pelagic eggs (i.e., ∼9 mm/s; Dudley & Platania 1999) is similar to that of fine sediment (Rubey 1933; Ferguson & Church 2004). In alluvial sand-bed rivers, fine sediments deposit on the surface of islands and bars (Nordin & Beverage 1964), so pelagic eggs must be deposited in a similar manner and develop on the surface of the substrate, as in a finger bowl. Also, Moore (1944) speculated that the swim-up behavior of protolarvae protects them from burial. Further, if burial threatened pelagic propagules, the prototypical behavior of spawning synchronously during high-flow events when the risk of burial is greatest (e.g., Moore 1944; Osborne et al. 2005; Durham & Wilde 2008) would be highly disadvantageous.

Pelagic-broadcast spawning could be a means to avoid destructive floods but take advantage of the flow-recession environment. Floods create nursery habitats, deliver propagules to them, and provide relief from predators and competitors (Moore 1944; Hoagstrom & Turner 2014). Retention occurs as discharge descends (Dudley & Platania 2007; Zymonas & Propst 2009), similar to fine sediments (Nordin & Beverage 1964). Recruitment occurs in the flow-recession environment (Moore & Thorp 2008; Durham & Wilde 2009a).

Although Wilde and Urbanczyk (2013) and Worthington et al. (2014) seem to view propagule displacement as important for pelagic-broadcast spawning, it could just be a by-product of strategies to enhance retention (Byers & Pringle 2006). Females produce two to three orders of magnitude more ova than are required to replace them and their mates (e.g., Wilde & Durham 2008; Durham & Wilde 2009b). Nearly all can drift away if a few are retained and at least one per parent survives to reproduce. Indeed, rivers on the plains have substantial retentive capacity (Dudley & Platania 2007; Zymonas & Propst 2009) and high fecundity, batch-spawning, flow-pulse-triggered spawning, and rapid propagule development all enhance propagule retention for pelagic-broadcast spawning minnows (Hoagstrom & Turner 2014).

Moreover, drifting larvae that must ‘move out of current’ on their own have a notable disadvantage. Larvae have minimal swimming ability, minimal energy reserves, relatively high metabolic energy demand, and limited sensory function (Fuiman 2002), so the odds of actively locating a suitable nursery before starvation must be relatively low. In contrast, individuals transported as eggs into suitable nurseries do not have to expend energy to locate them. Instead, they can hatch and develop rapidly in warm slackwater habitats and begin feeding at their convenience (Hoagstrom & Turner 2014).

Coincidentally, retentive river reaches are highly suitable for adults (Hoagstrom et al. 2008a, 2008b). Thus, spawning occurs where retention is high and young and adults commonly co-occur (Polivka 1999; Hoagstrom et al. 2006a; Durham & Wilde 2009a). Local retention seems evident where populations persist over time, spawning adults are present, and young-of-year outnumber adults (e.g., Lehtinen & Layzer 1988; Taylor & Miller 1990; Hoagstrom & Brooks 2005; Hoagstrom et al. 2008b). Young-of-year that recruit in proximity to adults also have the advantage of ready access to suitable juvenile and adult habitats (e.g., Polivka 1999; Hoagstrom et al. 2008a).

Wilde and Urbanczyk (2013) acknowledge that streamflow and habitat conditions in river fragments have ecological importance, but seem to conclude that fragment length is an overriding factor in pelagic-broadcast spawning minnow conservation. However, there is no evidence that fragment length alone has eliminated any population. Also, fragment length does not guarantee ecological suitability (Hoagstrom et al. 2008a, 2011). Extirpations occur in longer fragments, whereas shorter fragments sometimes sustain remnant populations (Dudley & Platania 2007). These shorter fragments are of particular interest because they provide opportunities to identify critical ecological features that complement fragment length.

At present, extinction risk, extinction debt, and restoration potential are poorly understood in most river fragments on the plains (Hoagstrom et al. 2011). All life stages are vulnerable and it is unknown which succumb to impacts that cause extinction. For instance, unnatural flow-regimes and channelization diminish propagule retention (Dudley & Platania 2007), but also reduce habitat suitability (Hoagstrom et al. 2008a, 2008b), and can disrupt dispersal (Lutscher et al. 2006). Streamflow intermittence precludes recruitment (Durham & Wilde 2009a), but also decimates adults (Hoagstrom et al. 2008a, 2008b). If vulnerable life stages and critical habitat features can be identified, future conservation and restoration efforts could be more focused. With so few occupied river fragments remaining, it would be unfortunate to miss research or conservation opportunities in any river fragment.

Acknowledgements

I thank R. Dudley and D. Walters for stimulating conversations relevant to this comment.