Evaluating removal methods of the invasive Brook trout and effects of removal on the ecosystem

The overall aim of the NOBROOK project is to develop new methods for removal of the invasive brook trout and to evaluate the effect of removal on ecosystems. Through establishing several control populations of known phenotypical distribution and a combination of brown trout in allopatry and in sympatry with brook trout be able to compare and test different removal methods across species and phenotypical traits. The response and recovery of the ecosystem after removal will we examined by investigating the composition of benthic invertebrates and fish diets as well as the potential impact presence of brook trout may have on the interaction between brown trout and the endangered freshwater pearl mussel.

The results will provide insights into the resilience of stream ecosystems and be helpful in efforts to restore and maintain the important ecosystem services and biodiversity connected to these habitats. This project is a part of The Swedish Environmental Protection Agency's management of invasive alien species program , and is divided into four work packages which you can read or hear more about below.

Workpackage 1

In the first WP the aim is to establish control populations and develop novel removal methods for brook trout.

In WP1 we will initially assess the allopatric and sympatric populations. These populations inhabit eight different natural stream sections we have selected in the Borås area of western Sweden. We will sample approximately 50 fish form each section (in sympatric sections we will aim for 25 of each species): two streams have allopatric brown trout and sympatric brown and brook trout populations at different sections (Ringsbäcken and Lindåsabäcken). Hökabäcken has sympatric populations upstream and downstream, and Sävbäcken has allopatric upstream and downstream. We will catch fish using electro fishing and traps. We will tag each fish with Passive Integrated Transponders (PIT) tags, weigh and measure fork length and photograph each fish. We will be assessing the social hierarchy by utilizing a modified open field freeze test and count opercular beat rates, and habitat use by monitoring their movement over time using portable antenna. We will also assess their diets by using stable isotope analyses of carbon and nitrogen through sampling of pelvic fin clips (isotope content in fins correlate with that of muscle tissue and allows for non-lethal sampling) and potential prey, and growth by recapturing the tagged individuals later in the season. These will form our “control” populations where we have data on growth, diets and boldness. We will also examine some fundamental properties of each section including temperature, pH, productivity and rate of decomposition and nutrient cycling, in addition to examining the benthic invertebrate compositions of our stream sections using kick sampling.

In the second part of this work package, we will test methods of removal. These methods include electrofishing, traps and nets. We will vary our electrofishing temporally and spatially in order to target more brook trout. Temporally this will be done at different times throughout the day as well as the season where we expect to find differences in capture rates and phenotypic traits depending on when we electro fish. We will also attempt to target brook trout where and when they spawn to reduce reproductive success. Different traps will be evaluated; “social” traps and traps baited with pheromones. In total, this work package will give use the comparable baseline information and the efficiency of several methods of removal.

Workpackage 2

In the second WP the aim is to study behavioral changes of both the invasive brook trout and the native sympatric brown trout (e.g. dispersal, trophic niche, habitat use) after removal/nearly removal of brook trout population.

Brown trout tend to alter their niche and habitat use in sympatry with brook trout. In this WP we will assess to what extent these effects remain after removal. We will also investigate whether these effects are equally impacted by brook trout, or whether a few more dominant or bold individuals affect brown trout behavior to a larger extent.

Workpackage 3

In the third WP the focus will be on evaluating the ecosystem after removal

Again, we will use the same stream sections as in WP1 allowing us to assess the composition of benthic invertebrates in the stream both in allopatric stretches and sympatric stretches as well as the composition of polyunsaturated fatty acids in brook and brown trout in sympatric and allopatric populations. The flux of emerging invertebrates will be quantified using emergence traps and their contribution to terrestrial consumers will be quantified using stable isotope analyses. We will also assess the stream section as they relate to functions including primary production, recycling of organic matter and nutrient cycling.

Workpackage 4

In the fourth and final WP we will focus on ecosystem effects as well; this time on the interaction between the endangered freshwater pearl mussel, brown trout and the invasive brook trout since all species often share habitats.

We will here focus on how brook trout and its phenotypic variability induced by removal may modulate interactions between a host (brown trout) and the parasite (larva of freshwater pearl mussel. Brook trout might have a direct negative effect on this relationship if brook trout gets infested but cannot produce juvenile mussels. However, there might also be indirect effects if the presence of brook trout would alter the temporal and/or spatial habitat use of brown trout resulting in fewer exposures to the glochidia larva. This WP will partly be performed in a different system than the other WP where we have a population of FPM.

To aim with WP4 is therefore to establish to which extent invasive brook trout can act as a functional host of FPM, and examine how this in turn affects the functionality of native brown trout as the main host for the glochidia larvae of the FPM, both direct and indirect.