Conservation value of Codium tenue habitat for the endangered Knysna seahorse Hippocampus capensis

Abstract

We investigated the density and size structure of the endangered Knysna seahorse Hippocampus capensis in the macroalga Codium tenue at three sites in a residential marina estate in the Knysna Estuary, South Africa, over an 18 month period from March 2017 to August 2018. Seahorses were present in C. tenue throughout the year. Average (± SE) densities of 0.33 (± 0.03) and 0.23 (± 0.03) seahorses per kg of C. tenue were found for 2017 and 2018 respectively. Density did not vary across sites but varied significantly across months with a decreasing trend from summer to spring. The average (± SE) adult seahorse height was 75.16 mm (± 0.63 mm) and 69.09 mm (± 0.64 mm) and mass was 2.05 g (± 0.04 g) and 1.65 g (± 0.04 g) for males and females, respectively, and an even sex ratio was observed throughout the sampling period. Height varied across months, with significantly smaller seahorses found in June and August of 2017. Tail length was highly correlated with height and therefore can serve as a potential proxy for determining the height of H. capensis. This study showed that C. tenue is consistently utilised by Knysna seahorse and should therefore be considered an important habitat to protect and facilitate the ongoing conservation of this endangered seahorse species.

Global extinction risk for seahorses, pipefishes and their near relatives (Syngnathiformes)

“…For the freshwater and estuarine pipefishes, in particular, conservation requires a combination of naturalizing flow regimes, addressing point-source pollution and nutrient influx, managing riverine and coastal development, and appropriately siting protected areas established within a whole-watershed framework. Such work is already underway in some areas, for example, in the Knysna Estuary, South Africa (de Villiers et al, 2019; Claassens et al, 2020).…”

Life history and population dynamics of an endangered seahorse (Hippocampus capensis) within an artificial habitat

“…The utilisation of this artificial habitat by an endangered species provides a novel conservation opportunity within this heavily modified environment and the habitat created by Thesen Islands Marina provides an opportunity for the application of reconciliation ecology as defined by Rosenzweig (2003). The possibility that these types of structures only act as aggregation devices (e.g., Baine, 2001;Grossman et al, 1997) Nonetheless, the priority would be to ensure that important natural habitats for H. capensis within the estuary, such as Z. capensis and Codium tenue (Bell et al, 2003;Claassens & Hodgson, 2018a;De Villiers et al, 2019;Lockyear et al, 2006;Teske et al, 2007), are protected and that anthropogenic disturbances are minimised.…”

“…A subsequent decline in natural habitats (soft corals and sponge habitats) as a result of anthropogenic (boat anchoring and moorings) and natural impacts (sand inundation) led to declines of over 90% of H. whitei populations in Port Stephens (Harasti, 2016). Past research on H. capensis noted population fluctuations which included a decrease in density from summer to winter observed within Codium tenue (Kützing) Kützing, 1856 beds (De Villiers et al ., 2019) as well as within Z. capensis and Reno mattress habitats (Claassens & Hodgson, 2018a). Changes in population abundances could be attributed to seahorse movement owing to breeding behaviour as seahorse species have been found to congregate (Harasti et al ., 2010) as well as disperse (Correia et al ., 2018; Freret‐Meurer & Andreata, 2008; Masonjones et al ., 2010) during the breeding season.…”

“…Nonetheless, the priority would be to ensure that important natural habitats for H. capensis within the estuary, such as Z. capensis and Codium tenue (Bell et al ., 2003; Claassens & Hodgson, 2018a; De Villiers et al ., 2019; Lockyear et al ., 2006; Teske et al ., 2007), are protected and that anthropogenic disturbances are minimised.…”

“…Previous research on H. capensis has focused on population abundances (Bell et al ., 2003; Lockyear et al ., 2006), range (Lockyear et al ., 2006) and habitat use (Claassens, 2016; Teske et al ., 2007). The Knysna seahorse is found within submerged vegetative habitats (De Villiers et al ., 2019; Teske et al ., 2007) and has adapted to withstand extreme and rapid salinity fluctuations (Riley, 1986), making it a true estuarine species. The Knysna estuary is considered the most important estuary for the long‐term conservation of this species (Teske et al ., 2003), as seahorse populations within the other two smaller estuaries are vulnerable to floods (Lockyear et al ., 2006) and mouth breaching (Russell, 1994).…”