The Importance of Sponge Gardens in Supporting Fish and Invertebrate Communities in Wellington Harbour

<p dir="ltr">With expansion in technology, humans are modifying the marine environment, often causing negative impacts on the abundance and resilience of marine organisms and ecosystems. Important habitats such as animal-dominated reefs (ADR) are declining in their extent and quality due to human influences, with consequences for millions of people who rely upon the functions and services that they provide. ADRs in the tropics, like coral reefs are well known to support abundant and diverse marine communities, but temperate and polar ADRs like sponge gardens are understudied and their value for broader ecosystem services is less well understood. This thesis investigates the ecological importance of sponge ADRs (sponge gardens), focussing on their role in supporting fish and invertebrate populations and their prey in Wellington Harbour, New Zealand. In Chapter 2, I quantify and compare fish and mobile invertebrate (>10mm) abundance and diversity inside sponge gardens and outside sponge gardens on adjacent bare habitat with the use of SCUBA surveys. Additionally, I assess the body condition of two dominant fish groups to evaluate whether sponge gardens contribute to healthier or stronger fish based on length-weight analyses. Results show that sponge gardens support on average, 10 times more fish and 3.2 times more fish richness than adjacent, bare habitats. Invertebrate abundance and diversity is also significantly higher inside sponge gardens, with 4 times more invertebrates and 1.4 times more invertebrate diversity than adjacent bare habitats. One of the dominant fish found in both habitats, <i>Tripterygiidae forsterygion</i> (Triplefin), show greater body condition inside sponge gardens than outside, whilst the most common wrasse species, <i>Notolabrus celidotus</i> (Spotty) shows similar condition regardless of habitat type.</p><p dir="ltr">In Chapter 3, I assess whether sponge gardens function to provide key food resources for fish by quantifying the volume and community composition of available fish prey (macroinvertebrates (<10mm)), inside and outside of sponge gardens, and comparing the stomach content of two dominant fish caught within and outside of sponge gardens, looking in particular for differences in prey preference. Sponge gardens hosted 10 species of macroinvertebrates unique to only sponge gardens and had significantly different prey community composition to adjacent bare habitats. Fish within sponge gardens consumed twice as many macroinvertebrate prey groups compared to those from adjacent bare habitats. Spotty and Triplefin diets were different inside sponge gardens and outside on adjacent bare habitats, with those outside showing a more diverse diet. This may suggest that prey availability or prey quality within sponge gardens is higher, leading to a narrower, more specialised diet in sponge garden associated fish. Prey preference is species-specific, with Triplefin preferring prey that can be sourced inside and outside the sponge gardens, while, Spotty associated with sponge gardens preferred Hymenosomatidae (crab), which was exclusive to sponge gardens, making up 97.84% of Spotty diet. This indicates that Spotty may be more dependent on the prey exclusively provided by sponge gardens, suggesting a stronger ecological link.</p><p dir="ltr">Overall, the findings of this thesis highlight the value and importance of temperate sponge garden ADRs for fish and invertebrates in Wellington Harbour, New Zealand. Sponge gardens support significantly greater abundance and diversity of fish and invertebrates with some species of fish being in better condition as well. Additionally, these sponge gardens support diverse fish prey with some fish species diet heavily relying on prey found only within sponge gardens. To ensure the biodiversity supported by sponge gardens is conserved, future management plans should prioritise the recognition, protection, and restoration of these important habitats, particularly in areas impacted by coastal development or seabed disturbance.</p>