DOES PREY SIZE OF LOLIGO FORBESI (CEPHALOPODA: LOLIGINIDAE) VARY WITH SQUID SIZE?

NICOLA JOHNSON

Department of Zoology, University of Aberdeen

Supervisor: Dr. Graham Pierce


e-mail:nicjohnson66@hotmail.com

Male Squid

Introduction

Materials and Methods

Results

Discussion

INTRODUCTION

The veined squid, Loligo forbesi, is commonly found throughout the Northeast Atlantic. It is a species commonly occurring in by-catch from commercial fishing vessels on the Scottish coast. However, this particular species is not part of an established fishery in Scottish waters.

Loligo is also classed as an annual species. In Scottish waters this means that they have a one year life cycle, with a single breeding season. However, typically of all cephalopods this particular species is an opportunistic feeder that will consume almost any prey taxa available . Loligo is a voracious predator and thus has evolved quite complicated and unique feeding apparatus.

All cephalopods have evolved a combination of muscular arms, tentacles and an intricately designed mouth.

Muscular arms have suckers running along them, these suckers assist the squid in seizing and enveloping the prey. The cephalopod mouth is situated inside the buccal mass. The buccal mass is a large and complex structure, consisting of the beak, the radula and the salivary glands. Within the mouth there is a chitinous beak used to chop up the food priorto ingestion. The whole process is assisted through the mechanism of the radula, which is also situated within the buccal mass. The radula is effectively the squid's 'tongue', it is a toothed strip that moves back and forth like a rasp. The radula assists in tearing the prey apart and passing food to the oesophagus within the mouth of the squid.

My study aims to examine whether there is a relationship between the size of prey and the increase of squid size in Loligo forbesi. The single most important way of examining this aim is to analyse squid stomach contents; however, this can cause a considerable number of biases:

  1. The major bias is a result of the squid's own behaviour.Loligo frequently rejects the head of their prey; thus the otoliths of fish are also discarded (Collins et al, 1996)
  2. The presence of secondarily ingested otoliths from the fish that the prey of the squid has already eaten (Collins et al, 1996).
  3. Lastly, due to the rapid rate of ingestion of the squid, (In Loligo it is between 3-6 hours), some remains may have already been digested before the squid was captured.

However, as highlighted earlier this is considered the best way to analyse diet and is a commonly recognised method to do so. Hence this study was based entirely on such a method. The aim of the study was, therefore, to investigate changes in prey size with changing size of squid.

MATERIALS AND METHODS
  • -During the period April 2000 to October 2000 samples were taken, on a monthly basis of the squid, Loligo forbesi. These samples were collected from commercial fishing landings at the very cold, wet and windy West of Scotland port of Kinlochbervie, as part of the CEC DG Fisheries study project 99/063, " Data collection for assessment of cephalopod fisheries". The five countries that collaborated in the study were: Portugal, Spain, Scotland, France and Greece.
  • -The monthly samples consisted of twenty individuals and contained a random mixture of both males and females (with the exception of October, which had a sample size of fifteen).
  • -Food remains were then washed with tap water over a sieve (300 microns) to separate the identifiable prey hard parts from the soft tissue remains.
  • -Otoliths were identified usually to species level using a guide under a binocular microscope and then measured.
  • -Vertebrae were then identified to the family level using a guide under a binocular microscope and then measured.
  • -The lengths of the fish from stomach content samples were then calculated from the otolith and vertebral lengths using the regression equations in the otolith guide and the bone guide.
  • -Separate regression slopes of fish lengths and Loligo forbesi mantle length were plotted for each individual species found in all the samples. This was carried out to test the null hypothesis stated earlier.
  • -Data was then put into an Analysis of Covariance (ANCOVA). This was done to examine the influence of month on the fish size and Loligo forbesi size relationship. The month was used as the model factor, fish length was used as the response variable and the squid mantle length was used as the co-variate.

-Finally, three Chi-square tests were carried out on the data.

1) The occurrence of each individual species was calculated and tested against the squid size categories.

2) Month was tested against the occurrence of individual prey species.

3) Lastly, a Chi-square test was carried out between the average squid sizes and the month.

RESULTS
  • - Crustacean remains were not identifiable to species.
  • - Otoliths and vertebrae were found in 82.2% of the stomachs. They represented seven fish taxa.

      The frequency of species found and the percentage ferquency is shown below:

      • Number of stomachs examined -135
      • Number of full stomachs - 40 (29%)
      • Number of empty stomachs -5 (4%)
      • Number of ¾ Full stomachs -28 (21%)
      • Number of ½ Full stomachs -41 (30%)
      • Number of ¼ Full stomachs -21 (16%)
      • Gadus morhua (Cod) -7 (5%)
      • Trisopterus minutus (Poor Cod) -17 (13%)
      • Trisopterus esmarkii (Norway Pout) -14 (10%)
      • Merlangius merlangus (Whiting) -25 (19%)
      • Unidentifiable Gadidae -68 (50%)
      • Ammodytes tobianus (Sandeels) -71 (52%)
      • Argentina sphyraena (Argentines) -3 (2%)
      • Loligo forbesi -1 (0.7%)
      • Sepiola -1 (0.7%)
      • Unidentified Sepidid -1UB
      • Unidentified crustaceans -12 (9%)

      Regression Analysis

      • Poor Cod - Significant
      • Unidentifiable Gadidae - Significant
      • All other species were found to be non-significant

      ANCOVA

      • Poor Cod - Significant
      • Norway Pout - Significant
      • All other species were found to be non-significant

      Chi-Square Tests

      Test One

      • All species were found to be signficant

      Test Two

      • All species except Norway Pout were found to be significant

      Test Three

      • Non- Significant, P-value= 0.859

      DISCUSSION

      General Diet Composition

      • - Diet composition is similar to that stated in the available literature.
      • - This study showed a diet of principally fish with crustaceans, molluscs and polychaetes taken at a lesser degree.
      • - Otoliths belonging to Argentines, Norway Pout, Whiting and Sandeels were found in the samples.

      Importance of Prey Species

      • - Previous studies have shown that there is a change in the diet of squid as they grow.
      • - Again, the findings of this study reflect the results of previous studies. It is clear from the results that the squid feed on smaller species like Sandeels and Argentines when they themselves are small and progress to larger Gadoid fish as they progressively grow.
      • - There are a number of explanations as to why Loligo feed more on fish rather than crustaceans and molluscs and to why they feed on the species of fish found in this study. Sandeels, were the most frequently occurring species in the results. There are a number of possible reasons for this, perhaps the most important is the calorific value of Sandeels compared to that of other fish species. Sandeels have a calorific value of 620 Cal/Ib compared to 500 Cal/Ib for Norway Pout and 350 Cal/Ib for Whiting and Cod. Therefore, it can be concluded that it would be highly beneficial for Loligo to feed on Sandeels whenever possible. However, this would only be effective if other prey species were not proportionally larger than the Sandeels.
      • - Furthermore, there are numerous possibilities and explanations as to why Loligo feeds on the six observed species. An important factor is that Sandeels, Argentines, Cod, Poor Cod, Norway Pout and Whiting are all demersal species and they all shoal, thus making them easier to catch.

      Effect of Time of Year on Diet

      • - There appears to be a significant relationship between the month and the squid-prey size relationship for Norway Pout. Poor Cod shows a significant relationship between squid length and prey length but shows a non-significant difference between the months. Moreover, there was no significant relationship for Whiting, Sandeels, Unidentified Gadidae, all Gadidae and all species found in the samples.
      • - There could be a number of reasons for the results found in this study. It could just be that the squid take a wider variety of prey species or that they might take prey available at the time of year [due to seasonal variation of the prey types].
      • - It can be concluded then that diets do change as the year progresses, probably having a significant relationship with the squid's increasing energy requirements as they grow and mature.

      Diet Choice and Prey Size

      • - The Chi-square tests were aimed to determine whether month and squid size had an effect on the occurrence of prey species. The results suggest that diet varies in relation to squid size for Poor Cod, Sandeels, Norway Pout, Whiting, Unidentified Gadidae and all Gadidae. It can also be concluded that diet also varies in relation to month for all species groups, except Norway Pout.
      • - However, since squid size generally increases from summer to winter it is possible that the differences that were seen, apparently related to size, are due to seasonal changes in prey availability. To explore this, the average squid size was tested against month.
      • - The Chi-square test showed a non-significant relationship, therefore it can be conclusively said that both month and squid size affects the diet.

      Dietary Theories

      • - The way squid feed is dependent on their growth stage. The more squid grow and mature, the more they feed and that the prey size increases. The present study appears to reflect these findings.
      • - A suggested reason for this increase in food size is that it reduces the energy expenditure of foraging. Thus, suggesting that the diet choice of Loligo forbesi is consistent with the optimal foraging theory. The theory states that 'foraging strategies may involve decisions which maximise the net rate of food intake, or of some other measure of foraging efficiency'. This could indicate that if a squid caught one large prey item rather than a selection of small items, it would be a better method of feeding. Clearly, this could be of great energetic gain to the squid.
      • - The diet of squid obviously depends upon the availability of prey. If Sandeels, for example, were more prominent in summer and of adequate size, Loligo could presumably feed on them. However, prey availability also depends upon such other factors as temperature, depth and geographic area. The dependency on prey availability has a great deal to do with the time of year. Fish are subject to seasonal variation in their numbers, therefore it could be assumed that squid feeding habits are also subjected to such seasonal variation. The present study reflects this, as the tests show a seasonal variation in squid diet.
      • - In conclusion, squid and other cephalopods in general are a unique group, due to their ability to grow and mature in such a short period of time with the mechanisms to feed efficiently and effectively. Cephalopods can thus occupy a wide range of ecological niches and trophic levels during their short life cycle. Therefore, Loligoforbesi could be 'free' to feed on a wide range of prey species.

      Limitations of Study

      • - In the present study a number of issues arose. Clearly Argentine and Cod samples were unable to be analysed effectively due to the limited size of the samples. They were far too small and not present in enough months. In order to undoubtedly prove the hypotheses a larger study would need to be undertaken using significantly higher populations of the prey species.
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