All flounder were measured to the nearest lower mm and grouped into one cm size classes between 1 to 6 cm. Naïve flounder were kept in 20 L plastic tanks in conditions similar to rearing conditions, and densities of ap- proximately 100 fish∙m⁻². Sea water flow of 1 L∙min⁻¹ was provided and the flounder were fed a diet of chopped mussels and commercial fish feed pellets during the daytime. Half the flounder were conditioned in the field in three aluminium bottomless cages (1.41 m × 1.41 m × 1.41 m) on a sandy substrate at a beach in Løgstør Broad, Limfjorden, Denmark. Conditioned flounder were held in these cages for three days at densities of approxima- tely 100 fish∙m⁻².

All turbot were measured to the nearest lower mm and grouped into one cm size classes between 1 to 7 cm. Na- ïve turbot were kept under similar conditions as the naïve flounder. The turbot were conditioned for three days in 20 L plastic tanks at densities of approximately 100 fish∙m⁻² with sufficient sand covering the bottom (3 cm) to allow burial behaviour.

The experiments were performed in round plastic tanks (h: 13 cm, d: 14.5 cm). Sediment was added to each tank covering the bottom by enough sand to allow burial (3 cm). The sediment was collected locally from an area, where both flounder and turbot naturally occur. The sediment was not sieved in order to make the test facility resemble a natural release site, although stones were removed. All tanks had an individual seawater flow of ap- proximately 100 ml∙min⁻¹. The water column in each container was 8 cm high with a volume of approximately 2 L.

A total of 62 flounder and 120 turbot were tested with 8 - 11 and 10 - 12 fish in each size group, respectively. Overhead strip lights and windows provided light for a photoperiod of 16:8 L:D. Temperature and oxygen levels were measured daily with an OxyGuard Handy Delta and salinity level was measured with a handheld refracto- meter.

One naïve or conditioned flounder or turbot (Exp. A1, A2, B1 or B2, Table 1) was gently released into a tank. Each fish was observed directly from above and the percentage of the fish’s dorsal surface covered by sediment was evaluated into one of four categories: 1 = not buried, 2 = less than 50% buried, 3 = more than 50% buried or 4 = completely buried. If the fish swam in the water column or adhered to the sides of the tanks this was noted. The behaviour was observed every five min during the first half hour and subsequently once every 30 min for a total of two hours. The tests were performed in daytime during the summer of 2007 and all fish were tested only once.

To test whether there was any effect of size on burying score (efficiency), the Kruskal-Wallis One Way Analysis of Variance on Ranks (KW) was used for the naïve and conditioned flounder and turbot.

The Mann-Whitney Rank Sum Test (MW) was used to test if there was an effect of the conditioning period of three days for the durations of 5 and 120 min, and for the total experimental period. The effect of time on the burying frequency of the flounder and turbot was tested using KW. All fish burying to any degree, scoring 2 or higher was included in these tests. All tests were run in SigmaStat (Version 3.5, 2006) and significance levels were set at 5%.

The temperature during the tests varied from 16.5˚C to 19.2˚C, with temperatures slightly higher in midsummer analysis than in the tests performed in late summer. The overall mean temperature was 17.6˚C. The mean oxy- gen content of the seawater was 7.3 mg O₂ l⁻¹ with measurements ranging from 5.6 to 8.0 mg O₂ l⁻¹. The salinity of the seawater was between 25 and 26 throughout the study with a mean salinity of 25.9.

Significant interspecies differences between the average burying time for flounder and turbot were found, with flounder burying more rapidly than turbot (Figure 1). This difference was especially pronounced for the smaller (size 2.0 - 2.9 and 3.0 - 3.9 cm), with flounder burying within 5 - 15 min while turbot buried after approximately 60 min. For fish larger than 4.0 cm, again flounder buried more rapidly at approximately 5 min while the turbot buried at 30 min (Table 2).

The frequency of buried flounder during the total experimental period divided into size classes is demon- strated in Figure 2(a). Except for the smallest group, all flounder buried quickly after release and the frequency of buried flounder generally increased with size. Time had no significant effect on the burial behaviour of flounder.

The number of burying turbot also increased with size (Figure 2(b)). However, in contrast to flounder, only a few of the turbot had buried at the beginning of the experiment. Time had a significant effect on burial behav- iour in turbot (KW, df = 8, P < 0.001).

For naïve 1.0 - 1.9 cm flounder, 63% of the fish never settled on the sediment while fish larger than 2.0 cm all settled for most of the observed period (Figure 2(a)). A similar pattern was observed for turbot (Figure 2(b)) in which 76 % of the naïve 1.0 - 2.9 cm fish never settled, but swam in the water column or adhered to sides of the tank during the observations.

The average burying scores for the first 5 min of the experiment (Figure 3(a)) and for the total observation pe- riod of 120 min for the laboratory experiments (Figure 3(b)) increased with increasing fish size for both naïve and conditioned flounder (KW, df = 2, P < 0.01 and df = 3, P < 0.05). The case was the same for naïve and con- ditioned turbot (KW, df = 4, P < 0.001 and df = 4, P < 0.005).

Conditioned flounder and turbot did not score higher or, in other words, they did not bury more efficiently than naïve flounder and turbot (Figure 3). Except for turbot 4 - 5 cm (MW, 120 min: P < 0.05, total period: P = 0.005), none of the conditioned fish buried more efficiently than naïve fish (see P-values in Table 3).

The evolution of flatfishes has been a gradual event [26] , and it is believed that their unusual morphology is an adaptation to avoid predation and ambush prey [27] . Burying to reduce predation risk has been demonstrated experimentally in sole [3] . Further, more summer flounder Paralichthys dentatus have been shown to bury in the presence of predators [28] .

Winter flounder did not bury immediately, concluding that for this species, burying is a learned response and not innate behaviour [3] . In contrast naïve flounder and turbot buried immediately in this study; a result similar to those of other studies [3] [29] who found that cultured turbot buried immediately and 99% of the reared sole buried in sand within one minute after release, respectively. In this study, no differences in burying behaviour between naïve and conditioned flounder and turbot were observed. Furthermore, the flounder and turbot used in

the present study showed clear signs of burying; e.g. alternated beating of the head and tail against the bottom of the rearing tanks (pers. obs). The same behaviour has been described as typical burying behaviour in Japanese flounder [30] . Along with the results of sole [3] , this suggests that while the tendency for certain flatfish to bury is innate, this may not apply to all flatfish as in the case of winter flounder [5] .

Of the 1.0 - 1.9 cm naïve flounder, 63% never settled on the sediment during the observations, but adhered to the sides of the tank or swam in the water column with a tilted body posture. This suggests that flounder settle when they are larger than 2 cm and eye migration is completed. For naïve turbot less than 3 cm, 76% never set- tled on the sediment. Hence, the turbot are larger than flounder when they settle. This is consistent with the fact that turbot are larger than flounder at metamorphosis, 16 mm and 8 - 11 mm, respectively [31] [32] .

This study demonstrated a difference between burying behaviour in different sized fish; both the frequency and efficiency of burying increased with size. A positive relationship between the size of the flatfish and its abil- ity to conceal itself has been demonstrated in other studies [22] [23] , and larger fish bury more efficiently in larger grain sizes than smaller fish did. It is assumed that larger fish bury more efficiently than smaller fish due to the greater force they can exert when concealing themselves [23] , suggesting that larger fish can inhabit more potential habitats than smaller fish [22] .

Conditioning fish before releasing into the wild is important for stocking success. The burial efficiency of simi- lar sized hatchery-reared fish increased to the same level as that of wild fish after a conditioning period of 12 days for sole [3] , and 2 days for winter flounder [5] . As we did not find any difference in burying efficiency between naïve and conditioned flounder and turbot in this study, we suggest that turbot and flounder may need longer conditioning periods than winter flounder. Several studies have demonstrated lower mortality rates [10] [33] with as much as a 50% decrease in mortality [11] . The improved survival in those studies may not be due solely to improved burying efficiency, but also to changes in feeding behaviour and thus lower vulnerability to predators [12] [34] . Further investigations are needed to clarify the specific causes for improved post-release survival rates in hatchery-reared fishes, but there is sufficient evidence to warrant the need for longer condition- ing periods for turbot and flounder before release.

The turbot did not bury as quickly as the flounder in this experiment suggesting an interspecies difference in burying behaviour. While 77% of all settled flounder buried to some degree within the first five min after release, only 16% of all settled turbot had buried within this time. Fish tend to demonstrate fright response caused by transfer stress when released into a new environment [35] , so the observed behaviour may have been a “stress response” even though precautions were made to stress the fish as little as possible before and during the release. The stress response of flounder is to bury rapidly while the stress response of turbot is to lie motionless on the sediment before eventually burying, and these behaviours have been observed previously by the authors.

Another possible explanation for the observed differences in burial efficiency is differences in diel feeding behaviour between flounder and turbot. However, there seems to be some disagreement on the diel activity of flounder and turbot in the literature (flounder nocturnal: [36] -[40] , turbot nocturnal: [39] [41] , flounder diurnal: [42] , turbot diurnal: [42] [43] ). These disagreements may originate from the fact that the flatfish change feeding habits depending on size, area, and available prey [40] [44] -[46] . The present experiments were carried out dur- ing daytime and if it is assumed that the flounder is nocturnal and the turbot is diurnal, this could cause the flounder to stay buried during daytime while the turbot would be more likely to stay on the sediment ready to pursue prey. Repeating this experiment at night using infrared light or examining diel feeding behaviour in flat- fish could help clarify this question.