"Conservation biology of the Spanish toothcarp (Aphanius iberus): mechanisms of coexistence with eastern
mosquitofish (Gambusia holbrooki)"

by Andrés Lucas Fernández Vilar

Supervision: Dr. Emili Garcia Berthou i Dr. Luis Miguel Zamora Hernandez

Ph.D. jury:

Dr. Anne E. Magurran, University of St Andrews

Dr. Carlos Fernández-Delgado, Universidad de Córdoba

Dr. Anna Vila-Gispert, Universitat de Girona

ABSTRACT

Inland waters are amongst the most diverse and impacted ecosystems globally, and they host many endangered taxa. Cyprinodontiforms are at the forefront of endangered freshwater fishes in Europe, with more than 80% of the species of this taxon considered threatened by the IUCN. The Aphaniidae, belonging to this order, are mostly distributed in the Mediterranean coastal areas, where ecosystems face many environmental pressures. This thesis consists of three main studies: (i) an analysis of the drivers and predictors of population extirpations of the Spanish toothcarp, Aphanius iberus; (ii) an adaptation and performance assessment of an underwater stereo-video system for studying the ecology of the fish assemblages of coastal lagoons, with emphasis on the endemic Spanish toothcarp and the invasive mosquitofish (Gambusia holbrooki); and (iii) a study, using the new stereo-video system, of diel and seasonal microhabitat partitioning between the endangered toothcarp and the invasive mosquitofish in relation to variation in limnological features.

For the first study, we compiled a database with the geographical locations of extant and extirpated Spanish toothcarp populations up to date and a number of associated environmental features. Multimodel inference and random forests analysis showed that habitat destruction, salinity and the human footprint are the most important predictors of extirpation. Decreased salinity has been well shown previously to favor invasive mosquitofish over toothcarp through several mechanisms. Accordingly, we found that the presence of this invasive species in low salinity habitats increases the odds of extirpation for Spanish toothcarp populations.

For the second and third main studies of this thesis, we adapted and tested a stereo-video system to study microhabitat use, size structure and fish species composition in coastal saltmarshes. With our setup, we obtained remarkable accuracy and precision in estimates of body length and depth positions (mean absolute errors of 5% and 3%, respectively). In this second study, we also deployed fyke nets simultaneously with stereo-video systems to compare results in species abundances and size structure of the fish populations. Our findings showed that stereo-videos provided similar results than fyke nets in species relative abundance and composition of the fish assemblages.

The third study showed that linear mixed models based on stereo-video data revealed a clear microhabitat use partitioning between the native and the invasive fish species, with mosquitofish occurring in shallower waters than the Spanish toothcarp, and both species shifting to deeper waters in winter. By contrast, their spatial niche overlapped in the early day hours in spring and summer as they engaged in aquatic surface respiration to cope with hypoxia. The endemic toothcarp displays resilience to hypoxic conditions in saline coastal refuges as it performs an adaptive behavior, revealed in this thesis, shifting from aquatic surface respiration to schooling in deeper waters alternatively. Our findings suggest that global warming may alter the odds of coexistence of toothcarps and mosquitofish as temperature raising might enhance anoxia and evaporation in shallow coastal lagoons and accelerate fish metabolism, among other factors.