Aqua-Feeds for Farming Fish – Chapter 2 Essentials

Feed Manufacturing & Formulation

• Feed types vary by manufacturing process, raw materials, and ingredient proportions.
• Two main production scales:
  • Farm-made aqua-feeds: local ingredients, supplemented with proteins/essential fatty acids, vitamin–mineral premixes.
  • Commercial dry feeds: crumbles, compressed pellets, extruded pellets; formulated via computerised least-cost linear programming.

Dry Feeds

• Moisture <10\% ⇒ easy transport, storage, consistent quality.
• Production methods:
  • Steam-compressed pelleting (dense, sinking).
  • Extrusion pelleting (float, slow-sink, harder, up to $\sim40\%$ fat).
• Extrusion uses higher temperature ($80–200\,^{\circ}\text{C}$) & moisture ($20–30\%$); needs fewer binders.

Life-Stage-Specific Feeds

• Broodstock: fortified with vitamins, minerals, specific fatty acids; aim = high-quality eggs, not growth.
• Larval: ultra-small (<400\,\mu\text{m}), microbound/microencapsulated, live food often required.
• Starter (crumbles): bridge from live prey; high surface area ⇒ fast disintegration.
• Grow-out: largest volume, high-energy (fat) or low-pollution (digestible, low-P) pellets.
• Finisher: pre-harvest; modify flesh colour (e.g., salmon), fatty-acid profile; may include probiotics.
• Medicated: contain drugs/antibiotics; must be highly palatable for sick fish.

Fishmeal & Fish Oil Dependence

• Key reasons: high-quality protein & source of $n!-<br /> 3$ highly unsaturated fatty acids (HUFAs).
• Global fishmeal production stable at $5–6\,\text{Mt yr}^{-1}$ since 1980s while aquaculture output >$!5\times$.
• Sustainability & contaminant (organochlorine, heavy metals) concerns drive need to reduce reliance.

Organic Contaminants in Feeds

• Dioxins, PCBs: persistent, lipophilic; higher in fatty fish, fish oils > fishmeals.
• Contaminant level depends on ingredient source, batch, and % fish oil in diet.
• Physiological effects: endocrine disruption (oestrogenic, anti-androgenic), immune suppression, carcinogenesis, developmental malformations.

Alternative Protein Sources

• Vegetable meals (e.g., soybean, canola) increasingly replace fishmeal; carp/tilapia/catfish diets often <5\% fishmeal.
• Limitations:
  • Amino-acid deficits (lysine, methionine) ⇒ require supplementation.
  • Antinutritional factors (ANFs) inc. phytoestrogens; mitigated by de-hulling, heat, solvent extraction.
  • High phytoestrogens may impair reproduction (reduced sperm motility, disrupted cycles).

Need for Marine Fish Oils

• Primary human dietary source of long-chain $n!-<br /> 3$ HUFAs (EPA $20{:}5\,n!-<br /> 3$, DHA $22{:}6\,n!-<br /> 3$).
• Supply limits & contaminant load prompt search for substitutes.

Plant Oils as Partial Substitutes

• Pros: low organochlorine levels, adequate fish growth.
• Cons: low $n!-<br /> 3$ HUFAs; fish flesh becomes rich in $18{:}1\,n!-<br /> 9$, $18{:}2\,n!-<br /> 6$.
• High $18{:}3\,n!-<br /> 3$ oils (linseed, canola) alleviate but do not replace long-chain HUFA needs.
• Research: transgenic plants engineered to produce long-chain $n!-<br /> 3$ HUFAs.

Mixed Feeding Strategy

• Grow fish on plant-oil-rich diets (low HUFA) ➔ switch to HUFA-rich marine-oil ‘finishing’ feeds before harvest.
• Advantages:
  • Flesh HUFA levels restored to “acceptable” values.
  • Lower lifetime organochlorine exposure.
  • More efficient use of limited marine oils.