Heavy metal accumulation in marine fishes in Porirua Harbour
Authors:   , Cook-Auckram Liana 
Published: December 2021

<p>Heavy metal or metalloids are common pollutants that are discharged into the aquatic environment by a variety of natural and anthropogenic sources, and have the ability to bio- accumulate in the tissues of marine organisms. Fish are among the top consumers in aquatic ecosystems and are widely recognised as bio-indicators for heavy metal pollution. Accumulation of heavy metals is influenced by factors such as species, age, size, and trophic level and can be found in various tissue types, such as muscle and liver tissue. In addition, contaminated fish can pose a threat to human consumers as they can cause acute and chronic disorders.  Estuaries are particularly vulnerable to heavy metal pollution as they are as they are a direct recipient of raw sewage, industrial, residential and farming runoff. Estuaries provide essential habitat for a range of species, including fishes that occupy estuaries permanently or seasonally for breeding. Te Awarua-o-Porirua Harbour (Porirua Harbour) is the largest, and the most significant estuary in the southern North Island of New Zealand. It is a 807 hectare tidal lagoon estuary next to Porirua City and consists of two distinct estuary arms, Onepoto and Pauatahanui. Porirua Harbour once boasted a healthy and diverse ecosystem that supported fishes that are prized by the Ngati Toa as kaimoana. However, heavy metal contamination has become problematic following the introduction of intensive industry and development in the harbour catchment.  The aim of this research was to 1) quantify levels of four heavy metals (Cu, Zn, Pb, and Hg) in the tissue (muscle and liver) of yellow belly flounder (Pātiki, tōtara, Rhombosolea leporina), sand flounder (Pātiki, Rhombosolea plebeia), speckled sole (Peltorhamphus latus), rig shark (Pioke, Makō, Mangō, Mustelus lenticulatus), short-tailed stingray (Whai, Dasyatis brevicaudata), and eagle ray (Whai keo, Myliobatis tenuicaudatus) caught in Porirua Harbour, and look for differences between sexes, tissue types, as well as effects of size and age, 2) examine each fish sampled for general metrics of health (parasite load, skin lesions, etc.) as well as diet, and look for relationships with body burdens of metals, 3) examine maternal offloading of heavy metals from pregnant rig shark to near-term embryos, 4) examine the movement of benthic fishes between the two estuary arms using mark/recapture methods.  To assess heavy metal accumulation and movement in benthic fishes, fish were collected and/or tagged over a 4-month period in 2018 (March-August) across 8 sites in Porirua Harbour. Tagged fish were unable to be recovered so conclusions were left undetermined. Overall, liver tissue had the highest levels of heavy metal concentration, with the expectation of Hg being elevated in the muscle tissue of rig shark. There were significant differences observed for species, fish size, with smaller fishes having higher Cu concentration, and larger fish having higher Hg concentrations. There was little to no relationship observed between Zn and Pb concentrations in this study.  To investigate the role of maternal offloading of heavy metals from maternal rig shark to their near-term embryos, embryos were collected from the uterus of 13 pregnant females and assessed individually for heavy metal (Cu, Zn, Pb, Hg) concentrations in muscle tissue. Overall, there was no relationship for Cu between the amount in embryos and either the maternal concentration or size. However, Zn and Pb concentration in rig shark embryos were positively related with maternal size. Therefore, size explained embryo Zn and Pb concentration in rig shark embryos, and embryo Hg concentrations were explained by maternal concentrations and size, suggesting maternal offloading of Hg might be occurring in rig shark.  The results of this thesis support prior research findings of heavy metal accumulation depending primarily on the tissue type, fish size and is metal and species specific. This research adds to the currently lacking information on heavy metal accumulation in these study species, and will aid the ongoing monitoring of Porirua Harbour by Greater Wellington Regional Council and Porirua City Council.</p>