Cumulative impacts of habitat fragmentation and the environmental factors affecting upstream migration in the threatened sea lamprey, Petromyzon marinus

Abstract

River ecosystems are often fragmented by artificial structures, such as weirs. For anadromous species, these structures can impede access to upstream spawning sites and ultimately lead to severe population declines. This study focused on the freshwater spawning migration of the sea lamprey, Petromyzon marinus, an anadromous species threatened by habitat fragmentation across its native range. To quantify the cumulative impacts of multiple weirs on upstream‐migrating adults, and to explore the environmental factors affecting migratory movements, passive acoustic telemetry was applied to 56 individuals during their spawning migration in the heavily fragmented River Severn basin, UK. While 89% of tagged sea lamprey passed the first weir upstream of the release site on the main river, only 4% passed the fifth weir. For 85% of migrants, the upstream extent of migration was immediately downstream of a weir. Individuals that passed weirs upstream of the release site (n = 50) took 21.6 ± 2.8 days to reach their most upstream location, experiencing cumulative passage times at weirs of 15.7 ± 2.8 days; these delays constituted a median of 84% of total upstream movement times. Multistate models showed that the weir passage rates of sea lamprey in tidal and non‐tidal areas increased significantly when downstream river level and discharge were elevated. Upstream‐to‐downstream changes in direction were frequent downstream of weirs, but rare in unobstructed river sections. The results provided evidence for a cumulative effect of multiple weirs on sea lamprey movements, substantially delaying upstream migrants and limiting their spawning to atypical habitat. The results also demonstrated the crucial roles of high tides and elevated discharge events in enabling weir passage. Although the Severn Estuary features conservation designations for sea lamprey, this study reveals that barriers are inhibiting their upstream migration, a problem that should be addressed to assist sea lamprey conservation.

Individual movement variation in upstream‐migrating sea lamprey Petromyzon marinus in a highly fragmented river

“…was increased during periods of elevated river discharge (Davies et al, 2021). Here, we revealed that across all of the lampreys passing Weir S2, there was a strong bimodal distribution in their passage times, whereby 32% of individuals passed within 24 hr of the first approach but the remainder passed after a median delay of over 2 weeks.…”

“…Passage time is a key metric for assessing the impact of barriers on upstream migration in fish (Silva et al., 2018), and numerous studies have identified the abiotic, individual, and behavioural factors affecting passage rates at barriers (Castro‐Santos et al., 2017; Goerig et al., 2020; Kirk & Caudill, 2017; Newton et al., 2018). A previous study on these lampreys indicated that their passage over Weir S2 was increased during periods of elevated river discharge (Davies et al., 2021). Here, we revealed that across all of the lampreys passing Weir S2, there was a strong bimodal distribution in their passage times, whereby 32% of individuals passed within 24 hr of the first approach but the remainder passed after a median delay of over 2 weeks.…”

“…Weirs S2 to S4, and T1, were the main river barriers under investigation here (Figure 1). Although they were all passable at high discharge levels, their permeability varied at other times; S3 and T1 were impassable at low discharge, S2 was passable at all discharge levels recorded during the study, and S4 was only approached/passed at high discharge (Davies et al., 2021).…”

“…To then decouple the causes of variation in passage times and retreat rates between intrinsic (body length) and extrinsic (environmental) factors, data from weir S2 were used as these provided the largest sample size of lamprey approach ( n = 56) and passage ( n = 50), with passage rates at this weir already established as being positively correlated with environmental variables, particularly increased river discharge (Davies et al., 2021). Lampreys that passed the weir were categorised as having been non‐delayed (passed the weir within 24 hr of arrival) or delayed in passage (passed the weir over 24 hr after arrival).…”

Predicting the influence of river network configuration, biological traits and habitat quality interactions on riverine fish invasions

“…Given the extensive fragmentation of rivers in Britain (Jones et al, 2019), and indeed in Europe (Belletti et al, 2020) and the world more generally (Grill et al, 2019), future work should consider incorporating barriers into network configurations, especially as these barriers can affect the behaviour of freshwater organisms (Rodeles et al, 2021;Starrs et al, 2017). However, this would require an additional algorithm relating to barrier permeability, such as the ability of fish to move upstream according to river level (Davies et al, 2021), and whether the barrier incorporated a fish by-pass structure (Pereira et al, 2017). Interactions of the invading fish with the native community were also not considered within the model, and it is acknowledged that these interactions can be an important factor in the outcome of biological invasions where, for example, the extent of native biotic resistance and resilience to the invader can determine the extent of its establishment and spread (Alofs & Jackson, 2014;Svenning et al, 2014;Thompson & Fronhofer, 2019).…”