New Zealand Sea Lions are large heavy-bodied sexually dimorphic animals. Adult males are 1.2-1.5 times longer and 3-4 times heavier than adult females.  Adult males are 2.1-2.7 m long and may weigh 300-450 kg (Geschke and Chilvers 2009).  Adult females are 1.8-2.0 m long and weigh 90-165 kg (Chilvers et al. 2005). Newborns are approximately 70-100 cm long and weigh 8-10 kg (Chilvers et al. 2006a).  Pups are born in a thick, long, dark brown lanugo with a lighter crown, nape, and mystacial area, and with a pale stripe on the top of the muzzle, originating on the crown. Female pups are lighter than male pups. Pups begin to molt their birth coat at two months old and at the end of the molt look like adult females.

Males become sexually mature at the age of five years. The age of maturity for females is 3-4 years. The average estimated reproductive rate of adult female New Zealand Sea Lions is 65% per year (Chilvers et al. 2010). Pup mortality at the end of one year was 30-55% for the Auckland Islands area (Chilvers and MacKenzie 2010), and pup mortality was 55% for the first 6 weeks after birth at Campbell Island (Maloney et al. 2012). Males live at least 23 years and females to at least 26 years (Reijnders et al. 1993, Childerhouse 2007).

The breeding season for the New Zealand Sea Lion begins in late November when adult males return and establish themselves on territories through displays, vocalizing, and fighting. Adult females arrive in early December and give birth on average within 2.1 days after returning to the rookery (Chilvers et al. 2006b). Males may have as many as 25 females within their territories. The bulls are frequently challenged by newly arriving males and neighbours, and turn-over of males is a regular occurrence. Many territorial bulls depart the rookery in mid-January with the end of the pupping period (Robertson et al. 2006).

The onset of oestrous occurs 7-10 days after a female gives birth. Prior to this, females continuously attend their newborn pup. Following mating, females begin a phase of short foraging trips followed by pup attendance, typical of many otariids. Foraging trips average 2.7 days and are followed by 1.5 days of pup attendance and feeding ashore (Chilvers et al. 2005). Pups gather into groups while their mothers are away. Pups are weaned at approximately 10 months. The primary causes of pup deaths within the first two months of life are trauma (35%), bacterial infections (24%), hookworm infection (13%), starvation (13%), and stillbirth (4%; Castinel et al. 2007a). Adult males are a source of mortality to pups, occasionally trampling them during territorial disputes and also through incidents of cannibalism.

New Zealand Sea Lions are not migratory, although males disperse widely over their range during the non-breeding season (Robertson et al. 2006). Some animals can be found at the major rookeries and haulouts year-round. At sea they are active divers that forage on both benthic and pelagic prey. Individual New Zealand Sea Lions have been found to have two distinct dive profile types or foraging patterns: a benthic diving profile and a deeper, more varied meso-pelagic diving profile (Chilvers and Wilkinson 2009). Mean dive depths for female New Zealand Sea Lions are to 129 m and mean dive duration is 3.9 minutes. Maximum dive depths are over 600 m and dives have been recorded to last as long as 14.5 minutes (Chilvers et al. 2006b). Mean total travel distances during foraging trips for lactating females are 423 km (SE = 43.9, max. = 1,087, n = 183). Satellite tracking data collected from 59 female New Zealand Sea Lions from all breeding sites at the Auckland Islands, indicates that they forage over the entire Auckland Island shelf, with extensive overlap with subantarctic trawl fisheries (Chilvers 2008, 2009; Chilvers et al. 2011).

New Zealand Sea Lions eat a wide variety of vertebrate and invertebrate prey. Frequently eaten species include Opalfish, Munida, Hoki, Oblique-banded Rattail, salps, octopus, squid, and crustaceans (Childerhouse et al. 2001; Meyneir et al. 2010). Antarctic, Subantarctic, and New Zealand Fur Seal pups and juveniles are taken as prey by adult male Sea Lions. Penguins are also occasionally taken (Lalas et al. 2007).

Great White Sharks are the only known predator of New Zealand Sea Lions (Robertson and Chilvers 2011).

New Zealand Sea Lions have a highly restricted distribution for a marine mammal. Their primary habitat is several subantarctic islands south of New Zealand, and their surrounding waters. The principal breeding colony is at the Auckland Islands, with the most of the remaining animals breeding at Campbell Island (Maloney et al. 2012). New Zealand Sea Lions regularly occur in small numbers at Stewart Island and on the southeast coast of the South Island of New Zealand, where there are some births (McConkey et al. 2002). However, most of the animals hauling out on the South Island are males ranging in age from 2 to 11 years old. Wandering New Zealand Sea Lions also reach Macquarie Island. Before human occupation (Maori and European), New Zealand Sea Lions had a more extensive range that appears to have included most of the New Zealand mainland and subantarctic islands. Polynesian midden records show pup and adult bones from the top of the North Island, through the South Island and into the subantarctic islands of New Zealand (Childerhouse and Gales 1998).

The New Zealand government has provided protection to New Zealand Sea Lions with laws that date back to 1881. The Marine Mammal Protection Act of 1978 added additional measures, stating that no marine mammal could be caught, killed, injured, attracted, poisoned, tranquillized, herded, harassed, disturbed, or possessed. However, those measures do not afford protection from incidental captures in commercial fisheries if they are reported to the appropriate officials as required. The uninhabited Auckland Fauna Reserve forms part of the habitat of New Zealand Sea Lions (Reijnders et al. 1993). Tourism is regulated on islands and at some mainland beaches on the South Island. Due to the declining population, the New Zealand Sea Lion was listed as a Nationally Critical New Zealand Species in 2010 under the New Zealand threat classification system (Baker et al. 2010, Townsend et al. 2008).

There are three main management strategies currently in place to mitigate New Zealand Sea Lion bycatch interactions in trawl fisheries off the Auckland Islands:

1) Input controls: a Marine Mammal Sanctuary and Marine Reserve surrounding the Auckland Islands extending 22.2 km offshore, within which no trawling or any other form of fishing is allowed. However, satellite tracking data indicate that this closure only protects a small part of the foraging areas of adult females (Chilvers et al. 2005, Chilvers 2009).

2) Output controls: restrict the number of New Zealand Sea Lions the trawl fishery may kill incidentally within designated fishery management zones before the zone is closed for the season (Chilvers 2008).

3) Sea Lion exclusion devices (SLEDs): SLEDs were introduced to the fishery in 2001. A SLED is a metal grid fixed inside the trawl net that allows smaller objects, such as squid, to pass into the cod-end, while larger objects are directed to an escape hatch opening. There is uncertainty about the efficacy of SLEDs and the overall impact of fishery interactions on New Zealand Sea Lion populations.