Long term trends

I investigated long term trends of main prey species in the diet of NZ sea lions. There is a higher

occcurence of red cod and rattail when comparing my timeframe (2000 – 2013) to Childerhouse et

al. (2001) diet data (1995 – 1997) (Figure 3-1). These species show a decrease in occurrence from

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targeted in the Auckland Island’s region (MPI 2013). However red cod is caught as by catch and also shows a corresponding increase in environmental availability (CPUE) over these years (2000-2013) (Figure 3-3).

Octopus species are not commercially targeted in the Auckland Islands region. In this study, giant octopus, and smaller octopus spp. were identified as being main prey species of NZ sea lions

(Chapter 2). The smaller Octopus sp. showed a clear increase in the diet from 1995 to 2013 (Figure

3-1). Giant octopus, however, appear to be slowly decreasing in the diet over time; if we consider the 2000-2013 timeframe, this decline is dramatic (Figure 3-1). The decrease in diet of giant octopus may indicate a shift from preying on large octopus which are energetically valuable, to smaller species that are less worthwhile. It is hard to quantify if this is due to a decline in abundance of giant octopus in the environment, or due to a behavioural shift. Octopus are not commercially harvested and are usually discarded when by-caught (MPI 2013). By-caught species are those that are caught incidentally by operations fishing for other species. The importance of octopus may reflect an increase in foraging effort as octopus are solitary animals that live in rocky areas on the sea floor of the Auckland Island’s shelf (Paul 2000; Meynier, Mackenzie et al. 2009).

Arrow squid are almost completely absent in Childerhouse et al. (2001) time period, and show large

temporal differences in occurrence from 2000-2013 (Figure 3-1). However, arrow squid appears to have shown a steep increase in the diet since 2009 (Figure 3-1). Arrow squid catch varies annually but very rarely meets total allowable commercial catch (TACC) (MPI 2013). The TACC is the total quantity of each fish stock that the commercial fishing industry can catch in a given year. The increase in the importance of Arrow squid in the diet since 2009 is a conservation concern for NZ sea lion since the Arrow squid fishery is the largest commercial fishing operation in this region (MPI 2013). Arrow squid is increasing in availability in the environment since 1999 after a decrease from 1995 to 1999 (Figure 3-3).

Hoki show a slight decrease in the diet over from time but this is highly variable with some years completely devoid of hoki in diet samples and others hoki is an important prey species (Figure 3-1). Hoki are the only fish species that is both one of the main prey of NZ sea lions and commercially targeted in the Auckland Islands region (MPI 2012). The commercial catch (CPUE) of Hoki has also been declining since 1999, which indicates that Hoki are less available in the environment, rather than a shift in prey preference (MPI 2013). Hoki may have been more prevalent in the diet historically when it was more abundant in the environment in the 1980’s (Figure 3-3). A similar drop

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in Hoki and increase in Arrow squid in diet of female NZ sea lions between 2000-2006, was identified by Meynier et al. (2010) using fatty acid analysis.

NZ sea lions are generalist feeders that prey on a variety of marine life, allowing them to adapt to changes in prey availability. I investigated whether NZ sea lions fed on main prey species in accordance with their abundance in the environment. Patterns of occurrence in diet of main prey species mainly tracked the abundance in the environment (Figure 3-3), with some differences. Deviation from this trend may be due to competition for resources, where sea lions are not able to take full advantage of abundant resources. Fisheries remove prey from the environment, potentially competitively excluding sea lions from taking prey that may have been initially abundant. Alternately, NZ sea lions may show preference for some prey types over others, meaning prey occurrence in the diet are not independent of one another. On the functional response curves, this would show a plateau at high abundance.

Fish species in general have higher energy content than cephalopod species, however this is not

significant (7.1 kJ g-1 compared with 6.3 kJ g-1 for squid; Meynier et al. 2008). The increase of

cephalopod prey in diet and concurrent decrease in fish species suggests a shift to lower quality diet (Chilvers 2012). My analysis of scat samples from 1995 to 2013 found that the ratio of fish:cephalopods in NZ sea lion diet has been decreasing with time (Figure 3-2). This indicates an overall reduction in diet quality. The arrow squid trawl fishery (SQU6T) operates on the Auckland Islands shelf from February til May (Chilvers 2009). This fishery removes substantial amounts of arrow squid, hoki and red cod from foraging areas of NZ sea lions during the lactation period, when breeding females and pups are at their most reliant on easily accessible and abundant food sources (Chilvers 2009). Reduction in prey availability/quality could impact upon female body condition, and therefore pup growth and survival (Augé, Chilvers et al. 2011). Changes in prey abundance and

quality have reduced reproductive output of Steller sea lions (Eumetopias jubatus) (Trites and

Donnelly 2003) and Southern elephant seals (Mirounga leonina) (Atkinson, DeMaster et al. 2008). In

addition, smaller prey types such as opalfish and Octopus sp. have been increasing while larger

species hoki and giant octopus have been decreasing in the diet over time (Figure 3-1). A reduction in these large prey types is likely to put further physiological stress on these animals. NZ sea lions are said to already be operating at their physiological limit, diving beyond their aerobic limits on most dives (Chilvers, Wilkinson et al. 2006). As such it is not possible for NZ sea lions to increase their energetic output to compensate for scarce or hard to catch prey.

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