Krill predator-prey interactions

The availability of krill to predators influences the foraging success of predators in two ways. Firstly, krill in deeper water are less likely to be detected by the krill predator species considered in this research (Tables 4.2 and 4.3). Secondly, many of the flying bird species are only capable of surface feeding or making brief dives in the top few metres of the water column, so krill deeper than this, even if detected, remain inaccessible. The mechanism for the detection of krill by land-based diving predators is unclear, but given the diving capabilities of species in this group, krill deeper than 80 m will typically remain

Predator Prey Explanation

Absent Absent Simply, neither predators or prey are present.

Predators have eaten prey to below a detectable threshold and departed. Unsuitable environment for prey

Absent Present Prey not detected by predators.

Predators have eaten some prey and departed. Predators foraging and diving so remain undetected from

the surface (availability bias).

Present Absent Predators in transit.

Predators searching for prey.

Predators engaged in social (non-foraging) activity. Prey eaten or dispersed.

Present Present Predators feeding.

Predators satiated.

Predators avoiding inter-specific competition. Chance overlap.

Table 4.1: Potential states of presence/absence in a marine predator-prey system, incor- porating air-breathing predators.

undetected. Consequently, examining the acoustically derived density of krill (ˆp) through the entire vertical observation range (z=250 m), as has been carried out in previous research (e.g. Hunt et al. 1992a and Reid et al. 2000b), may weaken the apparent spatial association between krill and predators.

In a 1998 survey, at the core box scale, Reid et al. (2000b) found a positive relationship between mean area krill density and the abundances of ten species of whale. This positive relationship weakened at smaller spatial scales, a result which was believed to be caused by a mismatch in the abundances of krill swarms and whales: there were more krill swarms than whales, so at smaller spatial scales fewer swarms were co-located with whales, thus weakening the overall spatial relationship between krill and whales.

4.1.3

Objectives

The objectives of this chapter are to use the contemporaneous krill density and air- breathing predator sightings collected during three multi-disciplinary research cruises be- tween 1997 and 1999 to: (1) determine the spatial scale of operation of krill and krill- predators at South Georgia across study sites and years, using a variety of techniques; (2) examine the implications of the differences in vertical distribution of krill for predator distributions; and (3) suggest how sampling on multi-disciplinary research cruises can be

improved to better examine the spatial relationships between predators and prey.

4.2

Materials and Methods

4.2.1

Sampling techniques

Line transect surveys comprising concurrent, continuous, hydroacoustic krill and visual air-breathing predator observations were conducted from the RRS James Clark Ross (JCR) in summers 1997, 1998 and 1999 in the vicinity of South Georgia, at two study sites, the WCB and the ECB (Figure 2.1, Chapter 1).

Predator observations

Times of encounter for all marine seabird and mammal sightings, along with predator species, group size and activity (e.g. feeding or transiting) were recorded continuously along each transect. A team of two researchers, one observer and one scribe, counted all species of seabirds and mammals encountered in a square with side length = 100 m, located 100 m in front of the JCR’s bow, which is effectively a strip-transect design, as described by Tasker et al. (1984) (Figure 4.1). For species in the divers and large flying bird groups (Table 4.2) only encounters where predators were observed to be foraging were used. All observations from the small flying bird group were included because it is difficult to identify foraging behaviour in these species.

During post-processing, the time of predator observation was used to assign the JCR’s GPS position (latitude: ϕS, longitude: λS) to each predator encounter. In this investiga- tion interactions between krill and air-breathing predators were assessed in post-processing over along transect aggregation intervals of 0.5 to 10 km. Within an aggregation interval the abundance of predators (n), or groups of predators, and the density of Antarctic krill (g/m2_{wet mass) was calculated. To avoid potential bias caused by the offset-JCR position}

being assigned to a predator encounter it was necessary to relocate the ship’s geographic position, as observed from the GPS antenna to calculate geographical position at the centre of the predator observation box (ϕP, λP, Figure 4.1). To adjust for the position difference caused by the along transect distance between the JCR’s GPS antenna and the centre of the predator observation box, the JCR’s GPS position assigned to a predator sighting was relocated by 175 m forward of the GPS antenna position, to the centre of the observation box using the geodetic inverse calculation. This calculation uses the observer geographic position, in this case the JCR GPS antenna (ϕS, λS), and the range (r=175 m) and bearing (θS, JCR heading) to determine the predator geographic position (ϕP, λP). The translation was performed using GeoCalc, v3.09 (Blue Marble Geographics).

Figure 4.1: Plan view of the predator observation square. The centre of the observation square (side length = 100 m) is located 100 m in from the bow of the RRS James Clark Ross, creating a strip transect with a width of 100 m. The ship position (ϕS, λS), deter- mined by the time of predator sighting was relocated 175 m along the current transect

θS, using the geodetic inverse calculation, giving the geographic coordinates at the cen- tre of the predator observation box (ϕP, λP) i.e. the predator’s true position at time of observation.

Krill observations

The spatial distribution and density of Antarctic krill was determined using active acoustic observations from a vertically downward looking, calibrated, EK500 scientific echosounder (Simrad, Norway) operating at 38 and 120 kHz frequencies with a ping repetition rate of 1 ping per 2.5 s, which at a nominal ship speed of 10 knots gave a ping spacing of 12.5 m. Acoustic data were post-processed enabling krill density to be described using two methods:

1. Grid method: krill density was calculated in discrete, equal along track intervals

and depths. A matrix of krill densities, with the spatial dimension of each element being 250 m along transect and 10 m deep, was created and the mean-variance relationship, correlation, spatial auto-correlation of krill and the cross-correlation of krill and predators were calculated. Krill densities derived using this method were also used to determine the vertical structure of krill in depth bands appropriate to the predator foraging depth.

2. Swarm method: Krill swarms were identified using the shoal analysis and patch

estimation system (SHAPES) as defined in Barange (1994) (see Section 2.2.3, Chap- ter 2 for methods).