Abstract
Aquaculture is the food production sector with the fastest growth. Research has shown that this industry can affect the marine ecosystem in different ways. It can impact the benthos, water column, and higher trophic levels. Some of these impacts haven been thoroughly studied, while others such as the direct and indirect effects aquaculture can have on cetaceans are not entirely understood. Only a few studies have been carried out on this topic and they have obtained different results. Some cetaceans increased, while others decreased their use of areas with aquaculture.
Regarding New Zealand species, not much is known about their interactions with this industry. Mortalities of Hector’s dolphins (Cephalorhynchus hectori hectori) have occurred in aquaculture farms, and two reports exist on this topic which highlighted the importance of carrying out more research before aquaculture expands further into the habitat of this endemic endangered dolphin species.
The aim of this study was to investigate potential effects of mussel farming on the marine environment from a broad ecosystem perspective. Different methodologies were utilised to assess the effects of this industry on the sediment, prey availability and the habitat use of Hector’s dolphins. Results showed effects at all these levels. Evidence of organic enrichment of the sediment was found, as samples collected at stations inside the mussel farms had significantly higher percentage of total organic matter compared to the stations outside the mussel farm. Regarding fish that are potential prey to Hector’s dolphins, there was a complex interaction between season and type of bay (with/without mussel farms) influencing the variation of fish biomass. In winter, there was significantly higher fish biomass in bays with mussel farms compared to bays without. Additionally, bays with mussel farms had significantly higher biomass of fish in the locations outside the mussel farm compared to inside.
Effects on Hector’s dolphins’ use of habitat were assessed through three different methods: passive acoustic monitoring with T-PODs, the analysis of a long-term (1988- 2020) sightings database and land-based tracking with a theodolite. Results from passive acoustic monitoring showed dolphin acoustic detections on at least 75% of days sampled in all the study bays. Results evidenced potential effects from mussel farms, with a much lower use of bays with farms in winter. On a finer scale, when T-PODs were placed in bays with mussel farms, dolphin detections were significantly lower on the mussel farm side compared to the opposite side of the bay. Additionally, data from the long-term sightings database showed a total of 174 dolphin sightings observed between the years 1988 and 2020 in the study bays. Data from the entire survey area in Banks Peninsula (from Sumner Head to Birdling’s Flat) showed that the dolphin sighting rate has increased over time. However, in the bays with mussel farms (MB, PB, PL), the sightings per trip (SPT) were significantly higher before compared to after the establishment of the mussel farm. Finally, land-based observations with a theodolite confirmed that Hector’s dolphins regularly used the area within a mussel farm in Pigeon Bay. A total of 269 dolphin fixes were collected, with 136 of them (50.56%) taking place within the mussel farm.
Even though results showed that Hector’s dolphins utilize areas with mussel farms, there is evidence suggesting potential impacts from this industry on this endemic endangered cetacean species. Therefore, it is suggested to conduct more research on this topic before the aquaculture industry expands further into the dolphins’ habitat.