travelled waters leads to a large number of ship strikes, with sometimes several of the whales dying each year from collisions with shipping vessels or sport boats. The population remaining is estimated to be between 100-200. In recent years, increases in numbers of migrating baleen whales are confirmed long after the end of hunting era. These are humpback whales, southern blue whales, pygmy blue whales, and southern minke whales. Less frequently, fin whales and sei whales are seen as well. For southern right whales, these whales will possibly become seasonal residents in the gulf as the populations recover (one of two of the first confirmed birth records on New Zealand's main islands since after commercial and illegal whalings were recorded at around Milford and Browns Bay in 2012). Sperm whales visit occasionally. Many of the islands are official or unofficial bird sanctuaries, holding important or critically endangered species like kiwi, takahe, brown teal and grey-faced petrel. Centred on the main conservation island of Tiritiri Matangi and Little Barrier Island, numerous bird species that were locally extinct have been reintroduced in the last decades, while there have also been some naturally occurring bird "re-colonisations", especially after introduced pests were removed from breeding and nesting grounds. Environmental damage The gulf is a vibrant natural environment, which has seen significant damage during the 20th and early 21st century from human use. Although major study by the Hauraki Gulf Forum in 2011 found that all environmental indicators were still worsening or stable at problematic levels, voluntary coast clean-up groups have collected about 450,000 litres of litter collected from the shoreline, although further conservation efforts are required to maintain the environmental integrity of the gulf. Particularly damaging were the introduction of industrialised fishing, with for example snapper fishing peaking in the 1970s at more than 10,000 tonnes a year (though even in the 2000s, private fishing of this species is also a considerable factor, weighing in at 400–800 tonnes a year). This severe overfishing, which unbalanced the marine environment by the removal of a main predator in the food chain, led to further degradation, such as a widespread disappearance of kelp beds as they were overtaken by kina barrens. Trawler fishing in general is seen as severely damaging the gulf, and lobster stock are also reported as not rebuilding. It is estimated that today's fish stocks are around 25% of pre-European levels. Also particularly damaging