Fish Feed Quality Is a Key Factor in Impacting Aquaculture Water Environment: Evidence from Incubator Experiments | Scientific Reports

Effects of different fish feed on nutrients concentrations without algae

Effects of different fish feed on phosphorus concentrations

Phosphorus is chemical compound found in fish feed33, its labile form (PO43−-P) is a major form of released phosphorus from fish feed43. From Fig. 1, TP, PO43−-P and TDP concentrations in treatments with HT, HP and ZT increase gradually in the first 10 days and then enter into a stable phase. Meanwhile, released concentrations of TDP and PO43−-P from fish feed reached 85.39~90.00% and 75.23~89.91% of their corresponding maximal values at the first sampling day (or 24 hours). Akhan and Gedik’s research results also indicated that release of nutrients from fish feed occurred rapidly, they believed that uneaten fish feed should be removed quickly to avoid nutrient enrichment32.

Figure 1

Variations of TP, TDP, TPP and PO43−-P concentrations with time in groups without algae (HT, fish feed of HT; HP, fish feed of HP; ZT, fish feed of ZT). Data shown is the mean ± SD of two independent measurements.

Full size image

Under same fish feed dosage, TP (TDP or PO43−-P) concentrations in treatments with HT and HP feed are 1.33~1.66 times higher than those of ZT, which is not consistent with their nutritional indicator of TP (in Table 1). This may be because the TP indicator in these feeds just follows “the lower limit rule”. Calculated results shows that average TP concentrations are 1.97, 1.96 and 1.28 mg L−1, average TDP concentrations are 1.75, 1.74 and 1.06 mg L−1, average PO43−-P concentrations are 1.60, 1.59 and 0.91 mg L−1 for HT 0.1 g, HP 0.1 g and ZT 0.1 g respectively, and these concentrations also doubles in treatments with 0.2 g correspondingly. This also implies that both HT and HP feed have much larger capacities in releasing phosphorus nutrients than ZT feed. In addition, significant analysis shows that there is a noteworthy difference in releasing phosphorus nutrients between HT and ZT and between HP and ZT (P < 0.001), while there is no significant difference between HT and HP (P > 0.05). Significant analysis also shows that fish feed dosage affects TP, TDP and PO43−-P concentrations quite significantly (P < 0.001), which conforms to Wu et al.’s results43.

In Fig. 1(b,d), variations of TPP concentrations with time are quite different from those of TDP. In general, TPP concentrations in HT, HP and ZT groups are quite low and close to each other with the same dosage of fish feed, and all increase firstly and then decrease slightly. Fish feed quality does not have a significant effect on TPP concentrations in general (P > 0.05).

Effects of different fish feed on nitrogen concentrations

Uneaten fish feed is probably the major input of nitrogen to the aquatic environment35,56,57,58, and the nitrogen cycle in aquaculture ecosystem begins with the introduction of protein in fish feed and NH4+-N is a by-product of protein catabolism26. From Fig. 2, compared with the released process of phosphorus nutrients from HT, HP and ZT fish feed, nitrogen concentrations rise comparatively very slowly and the time to reach nitrogen nutrients equilibrium concentrations is much longer. TN, TDN and NH4+-N concentrations increase gradually in about 15 days, and then reach equilibrium in the following days. In addition, it is clearly observed from Figs. 1 and 2 that TN equilibrium concentrations are higher than TP equilibrium concentrations (1.40~5.04 mg L−1) in the present experiment. Fernandes et al. also observed that leaching loads of fish feed for the bluefin tuna were slightly high for nitrogen as 26 kg N tonne−1, but significantly low for phosphorus as 4 kg P tonne−1 25.

Figure 2

Variations of TN, TDN, TPN and NH4+-N concentrations with time in groups without algae (HT, fish feed of HT; HP, fish feed of HP; ZT, fish feed of ZT). Data shown is the mean ± SD of two independent measurements.

Full size image

As shown in Fig. 2, released TN, TDN and NH4+-N concentrations from different fish feed are significantly different (P < 0.05): TN, TDN or NH4+-N concentrations with HT are the most, next with HP and the smallest with ZT; and actually these nutrients concentrations in the whole experimental period from HT fish feed are 1.17~1.52 times and 1.23~1.37 times the concentrations of HP and ZT, respectively for the same fish feed dosage. Average TN concentrations are 9.85, 7.20 and 5.36 mg L−1, average TDN concentrations are 7.93, 5.76 and 4.73 mg L−1, and average NH4+-N equilibrium concentrations are 6.63, 4.15 and 2.98 mg L−1 for HT 0.1 g, HP 0.1 g and ZT 0.1 g respectively, and corresponding concentrations with 0.2 g fish feed are almost twice their respective concentrations of treatments with 0.1 g fish feed. In reality, as shown in Table 1, ZT fish feed also contains the lowest crude protein, which may be due to the reason that ZT fish feed releases the smallest amount of nitrogen. In addition, similar to variations of TPP with time, TPN concentrations in Fig. 2(b,d) also fluctuate in low concentrations in all treatments during the whole period. Meanwhile, TPN concentrations are significantly different among the three different fish feed (P < 0.05).

As shown in Figs. 1 and 2, although the nutrients concentrations are significantly different in most experimental runs among HT 0.1 g, HP 0.1 g, ZT 0.1 g, HT 0.2 g, HP 0.2 g and ZT 0.2 g (P < 0.05), the nutrients’ proportions, namely, TDP:TP, PO43−-P:TP, TPP:TP, TDN:TN, NH4+-N:TN and TPN:TN, are quite close after all nutrients concentrations reach their equilibrium concentrations, as shown in Table 2, for example, TDP is 84.48~91.95%, 88.80~94.90% and 80.91~90.93% of TP for HT, HP and ZT respectively. From the results in Table 2, the ratio of PO43−-P to TP and NH4+-N to TN are obviously lower than those of TDP to TP and TDN to TN respectively because PO43−-P and NH4+-N are only one part of them, respectively. Proportions of PO43−-P and NH4+-N are in good agreement with Wu et al.’s results, and PO43−-P and NH4+-N have high proportions of TP and TN 43, respectively. Butz and Ven-Cappell59 and Kibria et al.35 also believe that fish feed contained major phosphorus fraction in a labile form, namely, the total phosphorus in fish feed, the more the water-soluble phosphorus. Thus, according to released P (TP, TDP and PO43−-P) and N (TN, TDN and NH4+-N) concentrations, we believed that HT contains the most nutrients, HP is next while ZT is the lowest in a comprehensive view. It is consistent with crude protein indicators of fish feed in general, ZT fish feed has the lowest crude protein level at 20%. Thus, based on trade-offs among feed price, feed efficiency, feed cost, feed quality, environmental impacts and so forth in aquaculture operations, we could improve protein bioavailability and design reasonable ratio of protein to energy to save protein and reduce nutrients emission.

Table 2 Nutrients proportions after nutrients reach their equilibrium concentrations.

Full size table

Effects of different fish feed on M. aeruginosa growth

Effects of different fish feed on M. aeruginosa densities

Fish feed contributes to abundant nutrient loads as discussed in the above, and it can effectively promote the growth of phytoplankton28,43,60. From Fig. 3, in the first few days of the experiment, algal cell densities increase very slowly due to their acclimation in fish feed medium with abundant nutrients in the medium. As time goes, M. aeruginosa cell densities increase very fast in the exponential phase (12~25 days) followed by a stable phase.

Figure 3

The growth of M. aeruginosa (MHT, M. aeruginosa + fish feed of HT; MHP, M. aeruginosa + fish feed of HP; MZT, M. aeruginosa + fish feed of ZT). Data shown is the mean ± SD of two independent measurements.

Full size image

Not only fish feed dosage but also their quality affects algae growth greatly, and the algae densities’ rankings in Fig. 3 are in agreement with those rankings of nutrients concentrations generally. The order of algae densities from the three different fish feed is MHT 0.2 g (MHT 0.1 g) > MHP 0.2 g (MHP 0.1 g) > MZT 0.2 g (MZT 0.1 g) during the whole experimental period (Fig. 3), and the corresponding measured maximum algae density is 2526.1 (1278.9), 2042.0 (1016.4) and 1757.2 (595.2) 1 × 104 cells mL−1, respectively. Two kinds of significant difference analysis of algae densities are conducted, namely, including and excluding lag phase, which indicates that the algae densities of MZT are significant different from those of MHT and MHP when excluding lag phase (P < 0.05), while they are not significantly different when including lag phase (P > 0.05), and this may be because the algae density is low and close to each other during the lag phase among the three different fish feed. In addition, fish feed dosage also has a significant effect on algae densities (P < 0.05).

Eutrophication is a major environmental problem induced by aquaculture activities, and algae densities reflect the level of eutrophication. Generally speaking, the lower the algae densities simulated by fish feed, the better the water quality is. Algae densities are coherent with released nutrients concentrations from fish feed and also consistent with nutritional indicators of fish feed in general. Thus, the above results imply that in order to protect aquaculture water environment, “environmentally friendly feed” are needed to both stimulate fish growth greatly and to lessen their effects on the water environment effectually in a balanced way. Meanwhile, new method is greatly needed to decrease the uneaten fish feed when throwing feed to fish manually and the uneaten fish feed also should be removed quickly before it releases nutrients to water.

In our study, both Fig. 3 and Table 3 show that the modified Logistic function can describe M. aeruginosa growth with good accuracy (R2 = 0.984~0.999) in agreement with the reported results49. Consistent with measured algae densities, \({N}_{max}\) and \({N}_{ave}\) (time-averaged algae density) of different fish feed are also in the order of MHT > MHP > MZT with the same fish feed dosage, and \({N}_{max}\) and \({N}_{ave}\) also increase with increasing dosages of fish feed. Specifically, the fitted Nmax are 2557.32, 2044.95, 1753.91, 1232.98, 979.49 and 593.59 1 × 104 cells (mL·d)−1 for MHT 0.2 g, MHP 0.2 g, MZT 0.2 g, MHT 0.1 g, MHP 0.1 g and MZT 0.1 g respectively, as shown in Table 3.

Table 3 Parameters of modified Logistic function describing algae growth. a, a constant; r (d−1), the intrinsic growth rate;

\({N}_{max}\)

(1 × 104 cells mL−1), the maximum algae density;

\({N}_{ave}\)

(1 × 104 cells mL−1), the average algae density;

\({R}^{2}\)

, square of correlation coefficient;

\({\mu }_{cmax}^{^{\prime} }\)

(1 × 104 cells (mL·d)−1), the maximal growth rate;

\({\mu }_{cave}^{^{\prime} }\)

(1 × 104 cells (mL·d)−1), the average growth rate;

\({\mu ^{\prime} }_{cmax}\)

(1 × 104 cells (mL·d)−1), the maximal specific growth rate;

\({\mu }_{cave}\)

(1 × 104 cells (mL·d)−1), the average specific growth rate. Data were calculated according to corresponding equations.

Full size table

Effects of different fish feed on the growth rate of M. aeruginosa

As shown in Fig. 4(a,b), both measured and computed growth rates in different groups all increase monotonously with time before they reached their maximal values, and then all decrease monotonously, which is consistent with Huang et al.’s study49. From Fig. 4(a,b) and correlation analysis, the computed growth rates agree reasonably well with measured ones with correlation coefficients (R) of 0.911, 0.954, 0.825, 0.970, 0.970 and 0.975 for MHT 0.1 g, MHP 0.1 g, MZT 0.1 g, MHT 0.2 g, MHP 0.2 g and MZT 0.2 g respectively, and all correlations are significant (P < 0.001). Although the analysis of significant difference shows that the fish feed quality does not have significant effects on growth rate (P > 0.05), maximal calculated growth rates (\({\mu }_{cmax}^{^{\prime} }\)) and averaged calculated specific growth rates of MHT are obviously the most, next those of MHP while those of MZT the smallest, as shown in Table 3.

Figure 4

Variations of growth rates and specific growth rates of M. aeruginosa in fish feed with time (MHT, M. aeruginosa + fish feed of HT; MHP, M. aeruginosa + fish feed of HP; MZT, M. aeruginosa + fish feed of ZT). Data shown are average value of two independent measurements.

Full size image

Effects of different fish feed on the specific growth rate of M. aeruginosa

Correlation analysis between measured and computed specific growth rates is conducted, the correlation coefficients (R) between measured and computed specific growth rates in all groups range from 0.713 to 0.841 (P = 0.002~0.037) except in group MZT 0.1 g with R = 0.579 and P = 0.188. This indicates that Eq. (3) is reasonably well in describing specific growth rates of algae generally. In Fig. 4(c,d), the computed specific growth rates increase firstly, then decrease in general. In addition, both measured and computed specific growth rates among different qualities’ fish feed are quite close with the same fish feed dosage, significant difference analysis also shows that fish feed quality does not influence the specific growth rates significantly(P > 0.05). This is because the specific growth rate is defined as the growth rate relative to (divided by) the algae density (as described in Eq. (3)).

Interaction of different fish feed and M. aeruginosa growth on nutrients concentrations

As discussed in 2.1, different quality of fish feeds has markedly different influence on released nutrients concentrations in general, that further affect algae growth. Wu et al. believe that in the presence of both algae and fish feed, nutrients releases were mainly controlled by fish feed dosage and algae utilization43. In the present study, not only fish feed dosage and algae utilization but also fish feed quality is taken into account to study the interaction of different fish feed and M. aeruginosa growth on nutrients concentrations.

Interaction of different fish feed and M. aeruginosa growth on phosphorus concentrations

Figure 5 shows variations of TP, TDP, TPP and PO43−-P concentrations with time in treatments with algae. From Fig. 5(a,b), some fluctuations of TP concentrations in treatments with algae were observed during the whole experimental period, and TP concentrations is not related to algae growth (R = −0.213~0.461, P = 0.072~0.928). Variations of PO43−-P concentrations with time are similar to those of TDP, and both concentrations decrease gradually to minimal values, which have negative relationships with M. aeruginosa growth (R = −0.965~−0.623, P < 0.010 for PO43−-P; R = −0.975~−0.539, P < 0.031 for TDP).The above variations of PO43−-P and TDP with time in the present study are consistent with Zhou et al.’s16 and Wu et al.’s43 studies.

Figure 5

Variations of TP, TDP, TPP and PO43−-P concentrations with time (MHT, M. aeruginosa + fish feed of HT; MHP, M. aeruginosa + fish feed of HP; MZT, M. aeruginosa + 0.1 g fish feed of ZT). Data shown is the mean ± SD of two independent measurements.

Full size image

The bioavailability of phosphorus depends on the phosphorus speciation, and algae take up phosphorus predominantly in the form of free orthophosphate35,61. Zhou et al.’s results also show that the dissolved reactive phosphorus (mainly PO43−-P) could be assimilated by algae at a higher velocity than other phosphorus forms17. As shown in Fig. 5, with the same fish feed dosage, any forms of P (TP, TDP and PO43−-P) concentrations in MHT and MHP are close to each other, which are higher than those of MZT. There are significant differences only between MHT and MZT as well as between MHP and MZT for TP concentrations and also there is a significant difference between MHP and MZT for TDP concentrations (P < 0.05). However, if we compare maximal and averaged TP, TDP, PO43−-P concentrations in the three different fish feed, they are actually quite different, and the most appears in MHT, and MHP is next while MZT is the smallest in general.

As shown in Fig. 5(b,d), TPP concentrations increase rapidly in the first 13 days then increase slowly in the following days. This is mainly related to initially released large quantities of phosphorus nutrients and uptake of PO43−-P nutrients by algae. In Huang et al.’s28 study, TPP concentrations are closely related to the algae biomass, namely, variations of TPP concentrations with time are similar to those of algae biomass. Correlation analysis in the present study also shows that there are positive correlations between TPP concentrations and algae densities in most groups (R = 0.710~−0.917, P < 0.002) expect group MZT 0.2 (R = 0.349, P = 0.192). This is because TPP concentrations do not increase and even decrease since day 11 in group MZT 0.2. Meanwhile, consistent with algae density, the order of TPP concentrations is also MHT 0.2 g (MHT 0.1 g) > MHP 0.2 g (MHP 0.1 g) > MZT 0.2 g (MZT 0.1 g), and the corresponding average TPP concentrations is 1.94 (0.89), 1.78 (0.70) and 1.47 (0.52) mg L−1. However, quality or dosage has no significant effect on TPP concentrations in general (P > 0.05), which maybe because the difference of algae density among different quality of fish feeds are not significant especially during lag phase.

In addition, it is needed to point out that TP includes both extracellular P and intracellular P in treatments with algae, thus variations of TP concentrations with time in treatments with and without algae should be similar. However, we noted that, influenced by algae utilization and algae deposition, TP concentrations in groups with algae fluctuate and are lower than those in group without algae43,62.

Interaction of different fish feed and M. aeruginosa on nitrogen concentrations

From Fig. 6(a,c), TN concentrations in treatments with algae increase gradually in the first 15 days and then keep stable in the following days, the variations are consistent with those in treatments without algae. Meanwhile, algae densities are also related to TN concentrations released from fish feed in general (R = 0.616~0.908, P < 0.011), while the correlation coefficients are low in group MZT 0.2 with R = 0.357 (P = 0.175). Fish feed quality has significant influence on TN concentrations (P < 0.05), and the order of TN concentrations in groups is MHT > MHP > MZT in Fig. 6. Maximal TN concentrations are 11.00, 7.56 and 6.09 mg L−1, the average values are 9.10, 6.09 and 4.57 mg L−1 for MHT 0.1 g, MHP 0.1 g and MZT 0.1 g respectively. Meanwhile the corresponding TN concentrations almost double in treatments with 0.2 g fish feed in general.

Figure 6

Variations of TN, TDN, TPN and NH4+-N concentrations with time (MHT, M. aeruginosa + fish feed of HT; MHP, M. aeruginosa + fish feed of HP; MZT, M. aeruginosa + fish feed of ZT). Data shown is the mean ± SD of two independent measurements.

Full size image

NH4+-N is the main form of TDN also being the preferred form of nitrogen for algae growth63. From Fig. 6, both TDN and NH4+-N concentrations in treatments with algae increase to their maximal values firstly which is mainly affected by the release of TDN and NH4+-N from fish feed, then decrease in the following days affected by algal nutrients utilization generally. In general, correlation analysis indicates that there are negative relationships between algae densities and TDN concentrations (R = −0.887~−0.369, P = 0.001~0.159) and between algae densities and NH4+-N concentrations (R = −0.867~−0.504, P < 0.046). Different from the results in treatments without algae fish feed quality observes no significant effect on TDN and NH4+-N concentrations among MHT, MHP and MZT (P > 0.05), except that there is significant difference of TDN concentrations between MHT and MZT. Whereas, maximal and average values also show that MHT contains most TDN and NH4+-N concentrations, MHP next while MZT contains the lowest. Actually, NH4+-N concentrations have dropped to almost 0 mg L−1 in treatments with 0.1 g fish feed in the later period of algae growth and to 0.33~0.38 mg L−1 in treatments with 0.2 g fish feed (Fig. 6(a,c)).

In Fig. 6(b,d), TPN concentrations increase gradually in the first 20 days and then reach stable concentrations with time going in MHT, MHP and MZT. Consistent with TPP, TPN concentrations also have positive correlation with algae densities during the whole experimental period (R = 0.744~0.920, P < 0.001). Also, the order of TPN concentrations at the same time among different treatments is MHT 0.2 g (MHT 0.1 g) > MHP 0.2 g (MHP 0.1 g) > MZT 0.2 g (MZT 0.1 g), and the corresponding average TPN concentrations are 10.95 (6.77), 9.30 (4.50) and 7.19 (3.61) mg L−1. However, fish feed quality has no significant influence on TPN concentrations among all treatments with algae (P > 0.05), and this may be also because fish feed has no significant influence on algae densities when including the data in the lag phase (P > 0.05, n = 16).

Due to the effect of algae growth, the fractional composition in treatments with algae, as shown in Figs. 5 and 6 and Table 2, is different from that without algae, as shown in Figs. 1, 2 and Table 2. For example, due to the algae utilization, the ratio of TDN:TN is 8.88~12.64%, 9.12~17.48% and 6.22~17.80% for MHT, MHP and MZT respectively (in Table 2), which are largely lower than those of HT, HP and ZT mainly because of selective uptake of nutrients by algae.

Effectsof different fish feed on nutrients utilization by M. aeruginosa

Nutrients releases from HT, HP and ZT fish feed are different as discussed in 2.1, which further affect algae growth and nutrients utilization. In order to study the interaction between different fish feed and M. aeruginosa growth, nutrients utilization by algae is also explored. In Huang et al.’s49 and Goudar et al.’s50 studies, Logistic function is also used to simulate nutrients consumption versus incubation time and as follows:

$$\Delta C=\frac{\Delta {C}_{max}}{1+{e}^{{a}_{\bigtriangleup C}-{r}_{\Delta C}t}}$$

(4)

in which t is the incubation time (d), ΔC (i.e. △TDP, ΔPO43−-P, ΔTDN and ΔNH4+-N) is consumed nutrient concentrations (difference of nutrients concentrations between without and with algae) at time t (mg L−1), \(\Delta {C}_{max}\) is the maximum consumed nutrient concentrations, \({r}_{\bigtriangleup C}\) is the consumed rate constant (d−1) and \({a}_{\Delta C}\) is a constant.

As shown in Fig. 7, △TDP, △PO43−-P, △TDN and △NH4+-N concentrations increase rapidly until it reaches their respective maximal consumed concentrations, then they remain stable. From Fig. 7 and Table 4, Eq.(4) can well describe variations of △TDP, △PO43−-P, △TDN and △NH4+-N concentrations with time (\({R}^{2}\) = 0.89~0.99), which is consistent with Kong et al.’s55 and Huang et al.’s49 study. In Table 4, it can also be founded that maximal calculated consumed TDP, PO43−-P, TDN and NH4+-N concentrations (\(\Delta {C}_{max}\)) and averaged measured consumed concentrations (\(\Delta {C}_{ave}\)) in different treatments are in the order of MHT 0.2 g > MHP 0.2 g > MZT 0.2 g > MHT 0.1 g > MHP 0.1 g > MZT 0.1 g, for example, the corresponding △Cmax of TDP is 3.85, 3.33, 1.99, 1.39, 1.38 and 0.75 mg L−1, respectively, this conforms to measured results. \(\Delta {C}_{max}\) increases with increasing maximum density of M. aeruginosa (\({N}_{max}\)), which indicates that more algae need more nutrients to grow (Fig. 7). Correlation analysis also shows that there is a positive correlation between algae density and consumed TDP, PO43−-P, TDN as well as NH4+-N concentrations with \({R}^{2}\) = 0.738~0.949, \({R}^{2}\) = 0.840~0.955, \({R}^{2}\) = 0.816~0.949, \({R}^{2}\) = 0.879~0.977, respectively. Meanwhile, fish feed quality has statistically significant effect on nutrient utilization if excluding the lag phase in general (P < 0.05) but no significant effect if including the lag phase (P > 0.05), and this is also because the algae density is close during the lag phase with different fish feed. In sum, the result implies that the nutrient utilization is dependent not only on the fish feed dosage but also on their quality.

Figure 7

Variations of consumed TDP, PO43−-P, TDN and NH4+-N concentrations with time (MHT, M. aeruginosa + fish feed of HT; MHP, M. aeruginosa + fish feed of HP; MZT, M. aeruginosa + fish feed of ZT). Data shown is the mean ± SD of two independent measurements.

Full size image

Table 4 Parameters in Logistic function of consumed nutrients concentrations.

Full size table

In Tijani et al.’s study, both nitrogen and phosphorus utilization display a significant increase during the first 2~21 days, then enter a stationary phase on the 21st day and the utilization has an initial 48 h lag phase64. However, in the present study, as shown in Fig. 7, algae have consumed 0~1.5 mg L−1 of P and 0~7.5 mg L−1 of N in the lag phase of algae growth, and the nutrients utilization do not show clearly a lag phase even if the algae densities are low. This may be because the algae in Tijani et al.’s64 experiment do not experience the starvation just before their experiments.