 Commercial fisheries can have both positive and negative effects on the greater crested tern . Juvenile survival rates are improved where trawler discards provide extra food , and huge population increases in the southeastern Gulf of Carpentaria are thought to have been due to the development of a large prawn trawl fishery . Conversely , purse @-@ seine fishing reduces the available food supply , and sizeable fluctuations in the numbers of great crested terns breeding in the Western Cape of South Africa are significantly related to changes in the abundance of pelagic fish , which are intensively exploited by purse @-@ seine fishing . Terns may be killed or injured by collisions with trawl warps , trapped in trawls or discarded gear , or hooked by longline fishing , but , unlike albatrosses and petrels , there is little evidence that overall numbers are significantly affected . 
