as many Calanus helgolandicus individuals per cubic metre, which were also found to be 50% longer. Unlike the megamouth shark and whale shark, the basking shark relies only on the water it pushes through its gills by swimming; the megamouth shark and whale shark can suck or pump water through their gills. Reproduction Basking sharks are ovoviviparous: the developing embryos first rely on a yolk sac, with no placental connection. Their seemingly useless teeth may play a role before birth in helping them feed on the mother's unfertilized ova (a behaviour known as oophagy). In females, only the right ovary appears to function, and it is currently unknown why only one of the organs seems to function. Gestation is thought to span over a year (perhaps two to three years), with a small, though unknown, number of young born fully developed at . Only one pregnant female is known to have been caught; she was carrying six unborn young. Mating is thought to occur in early summer and birthing in late summer, following the female's movement into shallow waters. The age of maturity is thought to be between the ages of six and 13 and at a length of . Breeding frequency is thought to be two to four years. The exact lifespan of the basking shark is unknown, but experts estimate to be about 50 years. Conservation Aside from direct catches, by-catches in trawl nets have been one of several threats to basking sharks. In New Zealand, basking sharks had been abundant historically; however, after the mass by-catches recorded in 1990s and 2000s, confirmations of the species became very scarce. Management plans have been declared to promote effective conservation. In June 2018 the Department of Conservation classified the basking shark as "Threatened - Nationally Vulnerable" under the New Zealand Threat Classification System. The eastern north Pacific Ocean population is a U.S. National Marine Fisheries Service species of concern, one of those species about which the U.S. Government's National Oceanic and Atmospheric Administration has some concerns regarding status and threats, but for which insufficient information is available to indicate a need to list the species under the U.S. Endangered Species Act (ESA). The IUCN Red List indicates this as an endangered species. The endangered aspect of this shark was publicised in 2005 with a postage stamp issued by Guernsey Post. Importance to humans Historically, the basking shark has been a