An ear for records : can otoliths unveil the early life history mysteries of freshwater fishes?

Abstract

Biological and environmental processes that influence the survival and fitness of individuals during their early life history phases can have implications for population structure and function. Historically, tracking early life history phase fishes has been problematic, due to their small size and fragility. Fortuitously, the earstones (otoliths) of bony fishes possess a suite of structural and chemical properties that can be analysed or exploited as a vector for chemical marking, to provide chronologically explicit information on the intrinsic biology and movement history of early life history phase fishes. In this thesis, I explore whether maternal injections of enriched stable barium and strontium isotopes are transmitted from an adult female Eastern rainbowfish Melanotaenia splendida to her offspring's otoliths, to provide a means of mass-marking larvae (transgenerational marking). I found that otoliths from larvae spawned by a female 6 months post-injection are marked. I conclude that transgenerational marking holds great promise for examining natal dispersal of rainbowfish in tropical stream networks. To examine if mixing two enriched stable isotopes can produce a third unique marker and whether transgenerational marking impacts upon larvae growth, survival or morphology, I use the tropical freshwater purple-spotted gudgeon Mogurnda adspersa as a model species. I found that I could produce a third distinguishable batch marker, and that transgenerational marking has no impacts on larvae survival or growth. A meta-analysis combining these results and previously published studies revealed that the impacts of transgenerational marking are non-significant, with the exception of negative growth effects in marine fishes injected with enriched barium isotopes. I conclude that transgenerational marking is rapidly coming of age and awaits wide-scale application. If transgenerational marking can facilitate the fine-scale tracking of individuals through space and time, can otolith microstructure reveal the growth history? I examine if the daily otolith increment widths of larval purple-spotted gudgeon otoliths accurately reflect the growth of larvae reared under variable but controlled environmental conditions. I found that backcalculation of growth using daily otolith increment widths was accurate under all situations except for during periods of food shortage and high temperature. This work suggests that backcalculation of growth using otoliths is a potent source of biological information, but must be applied cautiously. Otolith microstructure and chemistry can reveal the intrinsic biology and habitat use of individuals, respectively, but to what degree have these techniques been applied simultaneously? I performed a systematic review of 476 recent studies (2005-2012) employing otolith-based research to study the early life history ecology of fishes. I found that there is a clear split between studies focussed on biological objectives, which employ otolith microstructure analyses, and those studies exploring spatial objectives, with otolith chemistry analyses. A small number of studies (5%) utilise both the chemical and structural properties of otoliths to explore both biological and spatial objectives simultaneously. I contend that an integrated approach to otolith-based research exploiting both the biological and spatial information within otoliths should be a priority for future studies exploring the early life history mysteries of freshwater fishes. Show more