The first management of a marine invader in Africa: The importance of trials prior to setting long-term management goals

Abstract

Biological invasions severely impact on marine ecosystems around the world, but to date management is rare and has not previously been attempted in Africa. This study documents a trial management programme aimed at informing a national management strategy for the invasive European shore crab, Carcinus maenas, in South Africa. The approach involved testing control methods used elsewhere (baited traps, crab condos, diver collections and sediment dredging) and adapting these to the local context. Following these trials, baited traps were deployed over the course of the year, and the catch per unit effort (CPUE) tracked. A total of 36,244 crabs were collected during the management period, six times more than a pre-management population estimate. The population was not extirpated and CPUE increased once trapping ceased. The cost of attempting nationwide eradication is prohibitive, particularly given the lack of current impacts by this crab in this region and the possibility of reintroduction. We highlight key administrative challenges encountered, and the importance of such pilot trials in setting long-term goals when attempting alien species management interventions.

Molecular insights into the invasion dynamics of Carcinus crabs in South Africa

“…There are three possible explanations for the apparent stability of the Hout Bay population through time. First, the diversity and structure trends simply reflect the presence of juveniles in the population who were below the minimum catchable size during the period of management (Mabin et al 2020) that have since grown to a sufficient size to be retained in the traps. These juveniles would carry most, if not all, standing genetic variation from the previous generation.…”

“…The primary aim of that project was to assess the feasibility of eradicating Carcinus crabs from South Africa. However, approximately five times more crabs (36,244 individuals) were caught than the previously estimated total population size and extirpation was not achieved (Mabin et al 2020). This management trial also highlighted the need to understand the movement of individuals within and between populations that are targeted for management.…”

“…Samples were collected from native and invasive populations of Carcinus maenas and C. aestuarii around the world in 2015 (Table 1). South African samples were collected at Hout Bay and (Mabin et al 2020).…”

“…In recognition of the negative impacts of Carcinus crabs elsewhere in the world (e.g. Grosholz et al 2000, Matheson et al 2016, Ens et al 2022, and in support of authorities charged with managing biological invasions in South Africa, a management trial was implemented during 2014 and 2015 in Hout Bay harbour (Mabin et al 2020). The primary aim of that project was to assess the feasibility of eradicating Carcinus crabs from South Africa.…”

Double trouble: the implications of climate change for biological invasions

“…A major challenge will be to strengthen proactive response capabilities in countries that currently have low biosecurity capacity. The ability to meet this challenge will be intricately linked to capacity development in multiple fields including research, administration, and management (Mabin et al 2020). Such advancements in developing nations will be particularly important, as these countries are often particularly vulnerable to multiple drivers of change, including biological invasions.…”