Spelga Reservoir Fish Survey 2021

Date published: 25 May 2022

The survey was carried out by AFBI Fisheries Scientists in October 2021. It found the Spelga fish community consists of three species, brown trout, a sizeable three spined stickleback population and pike. The capture of a single pike in Spelga reservoir during this study is outlined, although in terms of Catch Per Unit Effort (CPUE) is currently at the lowest end of the scale when considering Irish lakes were both species are present.

Spelga has a relatively small surface area (55ha) and is generally shallow except for a small deep depression close to the dam wall, for these reasons the introduction of pike likely creates a vulnerable situation for the trout stock. At the time of this study electrofishing revealed good numbers of juvenile trout in the Spelga feeder streams however the netting survey in the reservoir failed to capture any trout < 3+ years of age. The inference here is that there is likely a high predation pressure on the juvenile trout once they reach the reservoir from the nursery streams. The individual pike examined was 60 cm at age 2+, this is quite fast growth for a typical unproductive high altitude waterbody.

At the time of sampling Spelga had a large population of small three spined sticklebacks which possibly facilitates the dietary shift at the lower end of the size range thus providing a high protein food source and consequential rapid growth at an early age. Conversely there is a building body of research from the Baltic region that describes the interaction between early stage pike and the three spined stickleback. Nilsson et al. (2019) found size selective predation on pike juveniles by three spined stickleback which suggest that it is crucial for pike to outgrow the stickleback predation window, their analyses further suggests that reaching a critical size of 80 mm will allow pike juveniles to start taking advantage of the significant resource constituted by high stickleback densities, which further manifest the importance of fast juvenile growth.

It is important to establish further information on the pike population in the reservoir as the timing of the survey when the water was cooler may have reduced the mobility of pike thereby lowering the catch in the nets. If it is established that pike are recruiting successfully, eradication may be difficult but management is likely achievable by intervention methods.

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