The parasite release hypothesis and the success of invasive fish in New Zealand

Non-indigenous species are commonly released from their native enemies, including parasites, when they are introduced into new geographical areas. This has been referred to as the enemy release hypothesis and more strictly as the parasite release hypothesis. The loss of parasites is commonly inferred to explain the invasiveness of non-indigenous species. I examined parasite release in New Zealand non-indigenous freshwater fishes. A literature review was undertaken in order to collate lists of the known parasite fauna of 20 New Zealand non-indigenous freshwater fish species. Records were collated from their home range, New Zealand, and some other introduced ranges, to determine whether these species have a reduced parasite diversity in the New Zealand and other introduced ranges. Five non-indigenous freshwater fish, mosquitofish (Gambusia affinis), rudd (Scardinius erythrophthalmus), goldfish (Carassius auratus), koi carp (Cyprinus carpio) and catfish (Ameiurus nebulosus), and one native freshwater fish, common bullies (Gobiomorphus cotidianus), were sampled and examined for metazoan parasites. Mosquitofish and bullies, of similar size and habitat, were examined with greater intensity than the other species. Based on the literature review and fish examination, I found that the non-indigenous freshwater fish in New Zealand have seemingly lost their parasites. Being “lost overboard” is likely to have caused the loss of some parasites as most of the additional hosts required for non-indigenous parasites are no present in New Zealand. However, the loss of most of mosquitofish’s parasites is likely due to “missing the boat”, as introduction into New Zealand was a two-step process, from their native range in North America, via Hawaii, before release in New Zealand. Additionally, other hosts required for parasites of mosquitofish are not present in New Zealand. The native common bullies were found to harbour more parasites than the non-indigenous mosquitofish. Two parasites were found from common bullies, the cysts of the trematodes Eustrongylides ignotus and Telogaster opisthorchis, and no parasites were found on mosquitofish. A lack of spillback of Eustrongylides ignotus from bullies to mosquitofish, despite being parasititised by this species in its native range, may due to mosquitofish not being able to feed on intermediate the hosts present in New Zealand. The establishment and spread of non-indigenous fish in New Zealand waters is likely not the sole result of parasite release, but given their apparent reduction or lack of parasites, it may be a factor contributing to their success

Some new developments in image compression

This study is divided into two parts. The first part involves an investigation of near-lossless compression of digitized images using the entropy-coded DPCM method with a large number of quantization levels. Through the investigation, a new scheme that combines both lossy and lossless DPCM methods into a common framework is developed. This new scheme uses known results on the design of predictors and quantizers that incorporate properties of human visual perception. In order to enhance the compression performance of the scheme, an adaptively generated source model with multiple contexts is employed for the coding of the quantized prediction errors, rather than a memoryless model as in the conventional DPCM method. Experiments show that the scheme can provide compression in the range from 4 to 11 with a peak SNR of about 50 dB for 8-bit medical images. Also, the use of multiple contexts is found to improve compression performance by about 25% to 35%;The second part of the study is devoted to the problem of lossy image compression using tree-structured vector quantization. As a result of the study, a new design method for codebook generation is developed together with four different implementation algorithms. In the new method, an unbalanced tree-structured vector codebook is designed in a greedy fashion under the constraint of rate-distortion trade-off which can then be used to implement a variable-rate compression system. From experiments, it is found that the new method can achieve a very good rate-distortion performance while being computationally efficient. Also, due to the tree-structure of the codebook, the new method is amenable to progressive transmission applications

Review of "Ajia" o Kangaeru 2000‒2015 (Thinking about "Asia," 2000-2015), Fujiwara Shoten editorial staff, ed., June, 2015.

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