The Continued Invasion of Northern New Zealand by the Asian Paddle Crab Charybdis japonica

Abstract

Invasive species are driving declines in global biodiversity. Brachyurans are particularly successful marine invaders. The Asian paddle crab, Charybdis japonica, is a large invasive portunid native to Asia, first detected in the Waitematā Harbour in 2000. However, little is known about the distribution, morphometrics, population demography, male influence on trap catch, where juveniles recruit, and the best method to capture them. Sampling across the North Island, in addition to fishermen interviews, were used to determine the crab’s current distribution. Morphometrics and population demography were analysed for C. japonica caught using traps. Male influence on trap catch and individual behaviours were examined using lab and field trials. Catch data from the Kaipara Harbour were examined to determine where juveniles recruit and the best method for detection. Charybdis japonica currently occurs from Ohiwa Harbour to the Bay of Islands on the east coast of the North Island and Manukau Harbour to Hokianga Harbour on the west coast. Analysis of distribution and invasion rate suggested that this dispersal is due to a range of vectors including international or domestic shipping, larval dispersal, aquaculture, and recreational vessels. The morphometric analysis revealed variation in interocular width between Kaipara/Manukau Harbours and Weiti River/Bayswater Marina and identified a possible morphometric size at maturity of males just below 60mm carapace width. The abundance of females varied seasonally, with all populations of C. japonica sampled being male-biased. The presence of large male C. japonica in traps decreased the probability of further captures through aggressive interactions, indicating that the male-biased catch may be a sampling artefact rather than being representative of the population present. Juvenile C. japonica recruit into separate habitats with less fine sediment when compared to adults within the Kaipara Harbour. Benthic sled tows were the most effective method trialled for detecting small juvenile C. japonica within the Kaipara Harbour. These findings have several management implications. To prevent the continued spread of C. japonica throughout New Zealand, increased biosecurity measures are required to prevent further regional spread. This includes monitoring and population control using multiple methods to target various life stages of C. japonica.