A novel hunting method for banded kōkopu

Abstract

Banded kōkopu (Galaxias fasciatus) were observed attempting to catch insects off the bank of the pool in which they lived. This is previously unrecorded behaviour for this species. Details of this behaviour were recorded and described when individuals responded to the placement of insect larvae on the bank of the pool. This behaviour could explain the differences between stomach and drift net samples in previous diet studies.

Keywords:

• banded kōkopu
• Galaxias fasciatus
• hunting behaviour
• New Zealand
• urban stream

Introduction

The banded kōkopu (Galaxias fasciatus Gray, 1842) is an endemic diadromus New Zealand fish species whose conservation status has changed over time; currently it is classified as ‘Not Threatened’ (Goodman et al. 2014). Banded kōkopu are considered to be primarily nocturnal (e.g. McDowall 1990; Halstead 1994, 1998; McCullough 1998; Baker 2000; Akbaripasand et al. 2014), but in some streams are crepuscular hunters (McCullough & Hicks 2002) with activity periods between 0400 h–0900 h and 1300 h–2100 h. Banded kōkopu do not see well in the dark (Halstead 1994) and much research has been done into their nocturnal prey detection abilities. They have been shown to detect and catch insects that have fallen into the water using their lateral lines (Halstead 1994) and sense of smell (Baker 2000). Terrestrial insects make up a large proportion of their diet, with overhanging streamside vegetation being an important source of these insects (Hicks 1997; Hancock 2009) which banded kōkopu catch in the stream drift (Main 1988; West et al. 2005; Akbaripasand 2012; Akbaripasand et al. 2014). These studies suggest that banded kōkopu are opportunists taking terrestrial insects as they drop into the stream or are washed into the pools from further upstream. However, there are differences in the abundance and types of insects caught in stream drift samples, and the numbers of insects reported in stomachs of banded kōkopu (Main 1988; West et al. 2005).

Here we report banded kōkopu actively hunting insects on the bank of the stream in which they live. This type of foraging behaviour has not been recorded or published before for this species.

Methods

Observations

Observations of approximately 30 banded kōkopu were carried out between 0700 h and 0815 h between 17 January and 8 February 2015 in a large pool (12 m², 0.5 m deep at the deepest point) on the upper reaches of the stream that runs through Le Roy's Bush, North Shore, Auckland (36^o48′56.81″S, 174^o44′21.87″E). Sizes of all fish in the pool were estimated visually and ranged from c. 100–200 mm total length (TL); with at least half of the individuals present being 100 mm TL or less. During this time we observed a number of individuals (all > 120 mm TL) jumping out of the water and snatching at items, predominantly where the stream banks were vertical.

In an attempt to better characterise this behaviour, we provided a food source on one of the two areas of the pool where we had seen kōkopu attempting to catch prey on the bank. On 7 and 8 February 2015 we initiated a series of trials using commercially available wax moth (Galleria mellonella) larvae. Larvae were easy to see and remained in approximately the same place when placed on the side of the pool. On 7 February 2015 between 0700 h and 0745 h, we placed one larva at a time on the eroded bank of the pool where we had seen banded kōkopu hunting. In less than 2 min, the larva was taken off the bank by a banded kōkopu in the manner we had observed before. On replacing the larva, the behaviour was repeated by the same fish. It was not possible to get accurate timing of the sequence of the behaviour because it happened too quickly but we managed to take some reference photographs.

Video of hunting behaviour

On 8 February 2015 between 0700 h and 0815 h, we repeated the exercise, but this time had a GoPro^TM 4 recorder placed strategically over the steep bank onto which we placed the wax moth larvae. The recordings captured on the GoPro were slowed down four times and the hunting behaviour analysed using the time clock on the recording. Attention was paid to the overall behaviour of the hunt and included when fish stopped during the approach to the prey or changed direction. Distance measurements were made from the video clips using Tracker^TM video analysis software and confirmed by measuring distances of identifiable objects in the pool with a tape measure. There are refraction considerations when locating and measuring distance through a water/air interface, which are complicated by the wide-angle lens of the GoPro camera, so distances away from the bank are approximate. One hunting sequence has been posted online (https://www.youtube.com/watch?v=Jbapl_E_n80&feature=youtu.be) and is presented as supplementary material to this short communication (Video S1).

Results

Observations

On 27 January we recorded two individuals, distinguished by size and pattern differences, taking up a position perpendicular to the side of the pool, and in the vicinity of an almost vertical bank created by high water flows (Fig. 1). The fish remained in this position for some time before moving to another place nearby and resuming the same position perpendicular to the bank. These fish launched themselves out of the water and onto the steep bank seven times in about 10 min. It was impossible to predict this behaviour in advance, so only opportunistic observations were made which were lacking in detail. Both individuals snatched at something on the bank but it was unclear whether they caught anything. During these hunts, their bodies were mostly out of the water and they fell back into the pool almost immediately.

On 29 January, at a similar steep bank, but on the opposite side of the pool, two individuals, based on size differences and body patterns, were observed to behave in an identical manner. The success or otherwise of their attacks was again unknown. On the same day a different individual was seen to launch itself out of the water and attempt to catch something off a dead palm leaf that was partially submerged. The outcome of this attack was also unknown. One of us (KH) visited the pool on 15 March and again on 18 April 2015. Larvae were provided on the bank and were removed by fish of a variety of sizes in an identical manner to that observed before. The smallest individual observed to jump out of the water was c. 120 mm TL; all other individuals were larger than this. At no time were any aggressive interactions observed with other fish, which might have prompted these individuals to jump out of the water.

Video analysis of hunting behaviour

Sixteen events in which a fish launched itself at prey situated on the bank were recorded and analysed. Fourteen larvae were provided on the bank in total. On detection of the larva on the bank, individual kōkopu swimming parallel to the shore immediately turned perpendicular to the bank (Fig. 1). In all instances this change in direction was followed by an attempt to remove the larva from the bank. Maximum detection distance was c. 25 cm from the larva.

Fish appeared to assess the position of the larva regularly and stopped an average of 3.44 times (range 2–4, n = 16) before capture was attempted. Distances moved between stops varied but were no more than 7 cm in any observed hunt. The last stop before capture was attempted was within 5 cm of the bank (Fig. 1). This distance appeared to vary according to the distance of the prey from the water's edge. The closer the prey to the water's edge, the closer to the bank the fish stopped before attempting capture. Individuals captured prey 14 times in 16 attempts. On the occasions that they were unsuccessful, the fish appeared to have misjudged the location of the prey. Mean attack time from detection (when the fish changed to a perpendicular position to the bank) to capture (when the larva was removed from the bank) was 9.84 s (range 2–26.5 s, n = 16).

Figure 1 Behaviour of banded kōkopu when taking prey (arrowed) from the stream bank. Three different fish (smallest > c. 120 mm total length) are approximately perpendicular to the bank and close to the water surface in A, B, and C. Prey (arrowed) is detected and attacked between 12.5 cm (A) and 3 cm (B, C) from the water surface. The fish launches from a stationary position and takes the prey from the bank (D). It then falls back into the water, where the prey is swallowed.

Some larger individuals swimming parallel to the bank and closer to the bank (15 cm) did not appear to see the prey. They actively harassed those fish that were hunting but only after the hunters had removed a larva from the bank. These large fish were not seen to attack the prey on the bank at any time but repeatedly attempted to remove the prey from the mouths of fish which had hunted successfully on the bank. No interactions were observed between any fish prior to the attack on the larva, on any occasion.

Discussion

This is the first time that banded kōkopu have been observed actively hunting prey on the banks of the pools that they live in. Not all fish in the pool were observed to hunt insects on the bank and it seems that this behaviour was undertaken by a few individuals, as determined by their stripe patterns (Hustler 2014) from the photographs that were taken. These individuals appeared to scan the bank regularly for prey capture opportunities and probably responded to any movement they detected.

Getting reliable data on this hunting behaviour by regular observation at the pool was almost impossible because of the difficulty of seeing potential prey (which could be seen by the fish but not by us) on the bank. The hunting behaviour could only be analysed by video, which had to be slowed down because of the speed at which the attacks occurred. Providing visible food items on the bank, which were eaten by banded kōkopu, allowed us to quantify some aspects of this behaviour, which would otherwise not have been possible.

It is possible that the fish have learned this behaviour, but not from our providing them with potential prey. They were observed jumping out of the pool and attempting to catch prey before we provided them with insect larvae. The only jumping we observed was directed at the larvae and nowhere else. The removal of streamside vegetation by contractors building a walkway for Auckland Council might have increased the frequency of this behaviour, but we saw fish behaving like this before removal of the vegetation.

We did not see any aggressive interactions between individuals of any size, which could have resulted in one of them leaving the water as a way of escaping the aggressor. The biting behaviour of the fish at the top of the jump, and when it made contact with the bank, suggested that it was attempting to remove something from the bank in its mouth. In all cases, interactions were only observed between individuals after the prey had been caught by one of them. Video S1 is typical of the hunting behaviour we saw.

Our findings suggest that the fish are actively monitoring the bank for prey-capture opportunities and that the behaviour we observed was hunting. During the visits in March and April, fish in the pool were observed to investigate newly created vertical banks caused by recent heavy rain. They swam parallel to the banks initially but turned perpendicular to them on a number of occasions. They then moved away after a short time only to take up another perpendicular position to the bank a short distance away. These fish were not observed to attempt to take prey off these new banks.

Anomalies between insects caught in the drift and the numbers reported in stomachs of banded kōkopu in the same area have been reported by Main (1988), who found that the most abundant prey in his study fish (coleopterans—28.7%) formed only 7.7% of items sampled in the drift. West et al. (2005) found terrestrial Amphipoda, Blattodea (cockroaches), Crustacea, Oligochaeta, Psocoptera (lice) and Neuroptera (lacewings) in banded kōkopu stomachs, which were not represented in their insect drift samples. Main explained this by suggesting that these insects were eaten before they were collected in the drift nets. Akbaripasand et al. (2014) report an energy deficit of up to 40% for banded kōkopu and state that the fish need to access other sources of energy in the form of benthic food items or terrestrial invertebrates to make up this deficit, but do not state how they might capture these. Foraging out of the water allows banded kōkopu to access other sources of energy from the habitat they occupy by actively hunting insects that venture close to the water's edge. This is a possible explanation for the differences observed between insect abundance in stream drift and in stomach samples taken from banded kōkopu (Main 1988; West et al. 2005). If this behaviour is shown to be more widespread, it suggests that we need to re-assess our classification of banded kōkopu as passive nocturnal drift feeders to active crepuscular hunters, under the right circumstances.

Associate Editor: Dr Jonathan Banks.

Supplementary data

Video S1. Video of typical sequence of banded kōkopu hunting behaviour.

Acknowledgements

Auckland Council gave permission to undertake these observations. Northcote College provided funding for the insects used in the experiment, which formed part of an NCEA Level 3 Biology internal assessment. John Blomfield assisted with data collection.

Disclosure statement

No potential conflict of interest was reported by the authors.