The introduced Micropterus salmoides in an equatorial lake: a paradoxical loser in an invasion meltdown scenario?

Abstract

Micropterus salmoides is a North American piscivorous fish on the IUCN list of 100 of the world's worst invasive alien species. Introduced into Lake Naivasha (Kenya) in 1929, their current population abundance is significantly depressed in a lake that has recently become dominated by fishes of the Cyprinidae family; the introduced cyprinid Cyprinus carpio now dominates catches in the commercial fishery and Barbus paludinous is now numerically dominant in the fish community. Long-term diet studies of M. salmoides based on gut contents analysis (GCA) have defined their diet spectrum, feeding preferences and ontogenetic dietary shifts. Between 1987 and 1991, diet was size-structured; fish \260 mm were mainly insectivorous and fish [260 mm fed mainly on invasive crayfish Procambarus clarkia with B. paludinosus rarely taken. More recent GCA data revealed that up to 2003, these sizestructured trophic relationships were still evident, but there has been a subsequent shift to their feeding almost exclusively on small (\100 mm) B. paludinosus, coincident with a size-related functional switch whereby M. salmoides [120 mm were now piscivorous. However, a Bayesian stable isotope mixing model (SIAR) suggested M. salmoides diet actually remained relatively varied in 2006 and 2007; it indicated P. clarkii were still contributing more to their diet than B. paludinosus in fish \260 mm and provided only partial support for the functional shift. The consequence of the M. salmoides depressed population abundance is their predation pressure on prey fishes is limited and preventing top-down effects. This is in contrast to their invasive populations elsewhere in the world and the likely result of invasion meltdown processes in Naivasha involving the introduced C. carpio and P. clarkii that have produced sub-optimal foraging conditions for M. salmoides.

A regional fish inventory of inland waters in Northern Italy reveals the presence of fully exotic fish communities

“…Albeit not a new hypothesis, it still remains controversial (Simberloff 2006) and very few examples of it are known from fish communities (e.g. the opposite effect found in Britton et al 2010). The ecosystem engineering capabilities of some of these exotic species could explain why these communities thrive.…”

Fish habitat suitability and community structure in the equatorial Lake Naivasha, Kenya

“…M. salmoides is a highly predatory fish and principally relies on water clarity as it forages by sight (Britton et al, 2010b). Their diet composition, which has recently shifted in Lake Naivasha from a crayfish-based diet to predominantly fish-based diet (Britton et al, 2010b;Oyugi, 2012), lends support to the ecological sustenance of discrete bass populations in rocky and deeper habitats characterised by clearer waters that assists their sight feeding (Britton et al, 2010b). Such environmental conditions were only available at Crescent Lake (CL) and Hippo Point (HP), resulting in their two disjunctive populations in the lake.…”

“…Consequently, previous studies on lake Naivasha have employed multi-mesh gillnets to provide relative measures of fish abundance using catch per unit effort (number of fish sampled per hour per gill net; cf. ; Britton et al, 2007Britton et al, , 2010b. Although not providing quantitative abundance estimates, through use of standardised gill net mesh sizes in each survey and sampling the same locations, these studies were able to infer temporal relationships in the relative abundance of the species in the fish community.…”

Temporal and spatial variation in fish assemblages in Lake Taihu, China

“…The increase in TN and TP over time has led to an increase in phytoplankton and zooplankton, resulting in an increase in small-sized phytoplanktivorous and zooplanktivorous fishes. Meanwhile, phytoplankton suspended in the water have reduced water clarity, affecting foraging of predatory fishes, which principally rely on water clarity because they forage by sight (Britton et al 2010). Thus, the relative importance of small-sized fishes (e.g., C. nasus) has increased rapidly because of the decrease in prey availability and fewer predatory fishes.…”